cloud: add new limitations on TiDB serverless for 7.1

[baiyuqing]

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[sunxiaoguang]

It's nice to have this note as cross reference.

Unfortunately, all the rest variables and statements did not have this. We should add them all or none.

[baiyuqing]

Agreed.

[qiancai]

If you search TiDB Serverless on https://docs.pingcap.com/tidb/dev/system-variables, you'll find there are more than 60 instances that indicate the TiDB Serverless support info.

For example:

Therefore, it looks OK to add the support info for this variable too.