ci: use jdk 21 instead of 8

Author: LunasaVT

.github/workflows/publish.yml

@@ -18,11 +18,11 @@ jobs:
       - name: Checkout repo
         uses: actions/checkout@v3
 
-      - name: Set up JDK 8
+      - name: Set up JDK 21
         uses: actions/setup-java@v3
         with:
           distribution: 'temurin'
-          java-version: '8'
+          java-version: '21'
 
       - name: Make Gradle executable
         run: chmod +x gradlew