When building on debian 12, we were using Java 17.
On debian 13, Java 17 is not packaged anymore, instead there is Java 21 and 25.
Ideally we should upgrade to Java 21 and just install it from apt.
However old Gradle is not compatible with new Java, so we have to upgrade Gradle for that.
- see https://docs.gradle.org/current/userguide/compatibility.html
Old Gradle is giving build errors with Java 21:
```
Could not compile build file '/home/user/wspace/electrum/.buildozer_qml/android/platform/build-arm64-v8a/dists/Electrum/build.gradle'.
> startup failed:
General error during conversion: Unsupported class file major version 65
java.lang.IllegalArgumentException: Unsupported class file major version 65
```
for our p4a fork, I tried to cherry-pick stuff from upstream:
- 17bf532279
- https://github.com/kivy/python-for-android/pull/3172a8f2ca1c5b
- see a01269f779...846a109487
That seems sufficient to upgrade Gradle as far as p4a is concerned.
However that still did not work yet:
- contrib/android/make_barcode_scanner.sh fails, as
markusfisch/zxing-cpp and markusfisch/CameraView
are also using too old gradle versions for Java 21
- it seems they are intentionally doing this to maintain compat with Android 4:
see d98ed5d0be
So for now maybe the path of least resistance is to downgrade to Java 17 :(
4da7b7f55d