cloud: add new limitations on TiDB serverless for 7.1#15645

baiyuqing wants to merge 4 commits intopingcap:release-7.1pingcap/docs:release-7.1from

baiyuqing:baiyuqing/serverless-limitation-7.1baiyuqing/docs:baiyuqing/serverless-limitation-7.1Copy head branch name to clipboard