ci: update JDK version to 21 for Gradle 9.x compatibility

Gradle 9.x requires JDK 17 or higher. Update both CI and publish
workflows to use JDK 21 instead of JDK 8.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Palbahngmiyine

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java-version: ['8', '21']
+        java-version: ['21']
     steps:
       - name: Checkout
         uses: actions/checkout@v4

.github/workflows/publish.yml

@@ -32,10 +32,10 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Set up JDK 8
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
-          java-version: 8
+          java-version: 21
           distribution: temurin
 
       - name: Setup Gradle