Bump all Java versions to 21 to fix GitHub workflow

Author: purejava

.github/workflows/publish-ayatana-maven.yml

@@ -18,7 +18,7 @@ jobs:
       - uses: actions/setup-java@v3
         with:
           distribution: 'zulu'
-          java-version: 20
+          java-version: 21
           cache: 'maven'
           server-id: ossrh
           server-username: MAVEN_USERNAME

.github/workflows/publish-minimal-maven.yml

@@ -18,7 +18,7 @@ jobs:
       - uses: actions/setup-java@v3
         with:
           distribution: 'zulu'
-          java-version: 20
+          java-version: 21
           cache: 'maven'
           server-id: ossrh
           server-username: MAVEN_USERNAME