gh-152785: Upgrade Java from 17 to 21 in GHA Android (#152783)

Upgrade Java from 17 to 21 in GHA Android, and document the Java version 
requirement.

Author: vstinner

.github/workflows/build.yml

@@ -354,7 +354,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Build and test
-        run: python3 Platforms/Android ci --fast-ci ${{ matrix.arch }}-linux-android
+        run: JAVA_HOME="${JAVA_HOME_21_X64:-$JAVA_HOME_21_arm64}" python3 Platforms/Android ci --fast-ci ${{ matrix.arch }}-linux-android
 
   build-ios:
     name: iOS

Platforms/Android/README.md

@@ -30,7 +30,8 @@ any packages it needs.
 The script also requires the following commands to be on the `PATH`:
 
 * `curl`
-* `java` (or set the `JAVA_HOME` environment variable)
+* `java` (or set the `JAVA_HOME` environment variable).
+  Java versions 17 and 21 are supported.
 
 ## Building