CI: build Gradle release job with JDK 21

The gradle-portal-push workflow set up JDK 17 and then ran `mvn install` on
multiapi-engine, which compiles with `release 21` — failing with
"release version 21 not supported". Bump the job to JDK 21, matching the Maven
workflows (which already build against 21). The engine and the Gradle plugin
both require Java 21.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: joseegarcia

.github/workflows/gradle-portal-push.yml

@@ -50,10 +50,10 @@ jobs:
         with:
           ref: ${{ github.head_ref }}
 
-      - name: Set up JDK 17
+      - name: Set up JDK 21
         uses: actions/setup-java@v2
         with:
-          java-version: "17"
+          java-version: "21"
           distribution: "adopt"
           server-id: ossrh
           server-username: MAVEN_USERNAME