Feature/use java21 (#31)

Switch to using Java-21.

.github/workflows/gradle.yml

@@ -22,10 +22,10 @@ jobs:
         fetch-depth: 0
     - name: Install graphviz
       run: sudo apt-get install graphviz
-    - name: Set up JDK 17
+    - name: Set up JDK 21
       uses: actions/setup-java@v3
       with:
-        java-version: '17'
+        java-version: '21'
         distribution: 'temurin'
     - name: Build with Gradle
       run: ./gradlew -Prepo.access.token=${{ secrets.REPO_ACCESS_TOKEN }} stage

.github/workflows/release.yml

@@ -31,10 +31,10 @@ jobs:
         registry: ghcr.io
         username: ${{ github.actor }}
         password: ${{ secrets.GITHUB_TOKEN }}
-    - name: Set up JDK 17
+    - name: Set up JDK 21
       uses: actions/setup-java@v3
       with:
-        java-version: '17'
+        java-version: '21'
         distribution: 'temurin'
     - name: Push with Gradle
       run: ./gradlew -Prepo.access.token=${{ secrets.REPO_ACCESS_TOKEN }} -Pdocker.registry=ghcr.io/${{ github.actor }} stage pushImages