chore: Update to Protobuf v4.31.0

Author: lqiu96

.kokoro/build.sh

@@ -62,7 +62,7 @@ javadoc)
 integration)
     mvn -B ${INTEGRATION_TEST_ARGS} \
       -ntp \
-      -Dprotobuf.version=4.31.0-RC1 \
+      -Dprotobuf.version=4.31.0 \
       -Penable-integration-tests \
       -DtrimStackTrace=false \
       -Dclirr.skip=true \