CI: revert to JDK 21

Author: manuelbl

.github/workflows/continuous-integration.yaml

@@ -26,11 +26,11 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: Setup Java
-        uses: oracle-actions/setup-java@v1
+        uses: actions/setup-java@v3
         with:
-          website: jdk.java.net
-          java-version: '22'
-          version: latest
+          distribution: 'zulu'
+          java-version: '21'
+          cache: 'maven'
       - name: Configure GPG key
         run: |
           echo -n "$GPG_SIGNING_KEY" | base64 --decode | gpg --import