ticdc: move the mysql downstream user permission info to the mysql sink doc

[qiancai]

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[LGTM Timeline notifier]

Timeline:

• 2024-07-29 06:59:37.922684937 +0000 UTC m=+166294.202733007: ☑️ agreed by Oreoxmt.

[qiancai]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #18185.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #18186.