ci: use Java 21 for Capacitor Android build (required by capacitor-android)

Author: j3k0

.github/workflows/build-capacitor-android.yml

@@ -20,7 +20,7 @@ jobs:
       - uses: actions/setup-java@v4
         with:
           distribution: 'temurin'
-          java-version: '17'
+          java-version: '21'
 
       - name: Setup Android SDK
         uses: android-actions/setup-android@v3