mac build: update README for new macOS 11 build VM

- For the notarisation, we were using "altool" (part of xcode), but Apple now fully deprecated that.
    - need to migrate from altool to notarytool
        https://developer.apple.com/news/?id=y5mjxqmn
    - currently using macOS 10.14.6
                  and xcode 11.3.1
    - notarytool requires xcode 13+
    - xcode 13 requires macOS 11.3
        https://en.wikipedia.org/wiki/Xcode#Version_comparison_table
    ==> created new build VM with macOS 11.7.10 and xcode 13.2

contrib/osx/README.md

@@ -11,7 +11,7 @@ This guide explains how to build Electrum binaries for macOS systems.
 
 This needs to be done on a system running macOS or OS X.
 
-The script is only tested on Intel-based Macs, and the binary built
+The script is only tested on Intel-based (x86_64) Macs, and the binary built
 targets `x86_64` currently.
 
 Notes about compatibility with different macOS versions:
@@ -22,40 +22,50 @@ Notes about compatibility with different macOS versions:
   imposes a minimum supported macOS version.
 - If you want to build binaries that conform to the macOS "Gatekeeper", so as to
   minimise the warnings users get, the binaries need to be codesigned with a
-  certificate issued by Apple, and starting with macOS 10.15 the binaries also
-  need to be notarized by Apple's central server. The catch is that to be able to build
-  binaries that Apple will notarise (due to the requirements on the binaries themselves,
+  certificate issued by Apple, and starting with macOS 10.15 (targets) the binaries also
+  need to be notarized by Apple's central server. To be able to build
+  binaries that Apple will notarize (due to the requirements on the binaries themselves,
   e.g. hardened runtime) the build machine needs at least macOS 10.14.
   See [#6128](https://github.com/spesmilo/electrum/issues/6128).
+  - There are two tools that can be used to notarize a binary, both part of Xcode:
+    the old `altool` and the newer `notarytool`. `altool`
+    [was deprecated](https://developer.apple.com/news/?id=y5mjxqmn) by Apple.
+    `notarytool` requires Xcode 13+, and that in turn requires macOS 11.3+.
 
-We currently build the release binaries on macOS 10.14.6, and these seem to run on
-10.13 or newer.
-
-Before starting, you should install [`brew`](https://brew.sh/).
+We currently build the release binaries on macOS 11.7.10, and these seem to run on
+10.14 or newer. (note: 10.13 might also work, haven't tested)
 
 
 #### Notes about reproducibility
 
 - We recommend creating a VM with a macOS guest, e.g. using VirtualBox,
   and building there.
-- The guest should run macOS 10.14.6 (that specific version).
+- The guest should run macOS 11.7.10 (that specific version).
 - The unix username should be `vagrant`, and `electrum` should be cloned directly
   to the user's home dir: `/Users/vagrant/electrum`.
 - Builders need to use the same version of Xcode; and note that
   full Xcode and Xcode commandline tools differ!
-  - Xcode CLI tools are sufficient for everything, except it is missing `altool`,
-    which is needed for the release-manager to notarise the `.dmg`.
-  - so full Xcode is needed, to have `altool`.
-  - however, brew now consider macOS 10.14 too old, and for some reason it
-    requires Xcode CLI tools. (`Error: Xcode alone is not sufficient on Mojave.`)
+  We use the Xcode CLI tools as installed by brew. (version 13.2)
 
-  So, we need *both* full Xcode and Xcode CLI tools. Both with version 11.3.1.
-  The two Xcodes should be located exactly as follows:
+  Sanity checks:
     ```
     $ xcode-select -p
-    /Users/vagrant/Downloads/Xcode.app/Contents/Developer
+    /Library/Developer/CommandLineTools
     $ xcrun --show-sdk-path
-    /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk
+    /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
+    $ pkgutil --pkg-info=com.apple.pkg.CLTools_Executables
+    package-id: com.apple.pkg.CLTools_Executables
+    version: 13.2.0.0.1.1638488800
+    volume: /
+    location: /
+    install-time: XXXXXXXXXX
+    groups: com.apple.FindSystemFiles.pkg-group
+    $ gcc --version
+    Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
+    Apple clang version 13.0.0 (clang-1300.0.29.30)
+    Target: x86_64-apple-darwin20.6.0
+    Thread model: posix
+    InstalledDir: /Library/Developer/CommandLineTools/usr/bin
     ```
 - Installing extraneous brew packages can result in build differences.
   For example, pyinstaller seems to pick up and bundle brew-installed `libffi`.
@@ -64,41 +74,12 @@ Before starting, you should install [`brew`](https://brew.sh/).
   (or run e.g. `git clean -ffxd` to rm all local changes).
 
 
-#### 1. Get Xcode
+#### 1. Install brew
 
-Notarizing the application requires full Xcode
-(not just command line tools as that is missing `altool`).
+Install [`brew`](https://brew.sh/).
 
-Get it from [here](https://developer.apple.com/download/more/).
-Unfortunately, you need an "Apple ID" account.
+Let brew install the Xcode CLI tools.
 
-(note: the last Xcode that runs on macOS 10.14.6 is Xcode 11.3.1)
-
-Install full Xcode:
-```
-$ shasum -a 256 "$HOME/Downloads/Xcode_11.3.1.xip"
-9a92379b90734a9068832f858d594d3c3a30a7ddc3bdb6da49c738aed9ad34b5  /Users/vagrant/Downloads/Xcode_11.3.1.xip
-$ xip -x "$HOME/Downloads/Xcode_11.3.1.xip"
-$ sudo xcode-select -s "$HOME/Downloads/Xcode.app/Contents/Developer/"
-$ # agree with licence
-$ sudo xcodebuild -license
-```
-
-(note: unsure if needed:)
-```
-$ # try this to install additional component:
-$ sudo xcodebuild -runFirstLaunch
-```
-
-Install Xcode CLI tools:
-```
-$ sudo rm -rf /Library/Developer/CommandLineTools
-$ shasum -a 256 "$HOME/Downloads/Command_Line_Tools_for_Xcode_11.3.1.dmg"
-1c4b477285641cca5313f456b712bf726aca8db77f38793420e1d451588673f9  /Users/vagrant/Downloads/Command_Line_Tools_for_Xcode_11.3.1.dmg
-$ hdiutil attach "$HOME/Downloads/Command_Line_Tools_for_Xcode_11.3.1.dmg"
-$ sudo installer -package "/Volumes/Command Line Developer Tools/Command Line Tools.pkg" -target /
-$ hdiutil detach "/Volumes/Command Line Developer Tools"
-```
 
 #### 2. Build Electrum