Change Java CI workflow to JDK 21

Updated Java CI workflow to use JDK 21 and improved build steps.

Author: QuittoGames

.github/workflows/maven.yml

@@ -0,0 +1,30 @@
+name: Java CI with Maven
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: '21'
+          cache: maven
+
+      - name: Build with Maven
+        run: mvn -B clean package
+
+      - name: Check Java version
+        run: java -version
+
+      - name: Update dependency graph
+        uses: advanced-security/maven-dependency-submission-action@v3