GitHub Actions: also setup JDK 21 for gomobile

.github/workflows/release.yml

@@ -30,6 +30,12 @@ jobs:
         with:
           go-version: 1.23.4
           cache-dependency-path: "**/go.sum"
+      - name: Setup Java
+        uses: actions/setup-java@v4
+        if: steps.cache.outputs.cache-hit != 'true'
+        with:
+          distribution: 'temurin'
+          java-version: 21
       - name: LibCore Cache
         id: cache
         uses: actions/cache@v4