ci: Revert to Java 17 due to 21's Mockito Issue in storage-nio

Author: lqiu96

.github/workflows/downstream_protobuf_compatibility_check_nightly.yaml

@@ -37,7 +37,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-java@v4
         with:
-          java-version: 21
+          java-version: 17
           distribution: temurin
       - name: Print Protobuf-Java testing version
         run: echo "Testing with Protobuf-Java v${{ matrix.protobuf-version }}"