contrib: rename some shell scripts to have ".sh" extension

The extension gives formatting hints to some editors. (especially if they support .editorconfig)

contrib/osx/README.md

@@ -103,17 +103,17 @@ $ hdiutil detach "/Volumes/Command Line Developer Tools"
 #### 2. Build Electrum
 
     cd electrum
-    ./contrib/osx/make_osx
+    ./contrib/osx/make_osx.sh
 
 This creates both a folder named Electrum.app and the .dmg file.
 
 If you want the binaries codesigned for MacOS and notarised by Apple's central server,
-provide these env vars to the `make_osx` script:
+provide these env vars to the `make_osx.sh` script:
 
     CODESIGN_CERT="Developer ID Application: Electrum Technologies GmbH (L6P37P7P56)" \
     APPLE_ID_USER="me@email.com" \
     APPLE_ID_PASSWORD="1234" \
-    ./contrib/osx/make_osx
+    ./contrib/osx/make_osx.sh
 
 
 ## Verifying reproducibility and comparing against official binary