Update protobuf.version to 4.33.0

Author: lqiu96

.kokoro/build.sh

@@ -61,7 +61,7 @@ javadoc)
     ;;
 integration)
     mvn -B ${INTEGRATION_TEST_ARGS} \
-      -Dprotobuf.version=4.32.1 \
+      -Dprotobuf.version=4.33.0 \
       -Penable-integration-tests \
       -DtrimStackTrace=false \
       -Dclirr.skip=true \