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

Upgrade Java from 17 to 21 in GHA Android, and document the Java version
requirement.
(cherry picked from commit 0a13efc3fb0bf0896612ed20decce3ce4eb5e05b)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: vstinner

.github/workflows/build.yml

@@ -401,7 +401,7 @@ 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