ds-java main branch requires 21

Author: AlexanderSaydakov

.github/workflows/serde_compat.yml

@@ -21,7 +21,7 @@ jobs:
       - name: Setup Java
         uses: actions/setup-java@v4
         with:
-          java-version: '17'
+          java-version: '21'
           distribution: 'temurin'
       - name: Run Java
         run: cd java && mvn test -P generate-java-files