ticdc: Update migrate from tidb to tidb doc

[sdojjy]

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[qiancai]

这个注意提到的是 “在启动 TiCDC server 时指定 gc-ttl 的值”，但是 L149 这里还没开始部署 TiCDC，需要把这个注意挪到下面这里吗

[sdojjy]

可能放在这里更加合适一些？ 因为主要是提醒用户 默认情况下如果备份恢复时候过程可能创建不了 changefeed

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-23 05:46:22.765433976 +0000 UTC m=+503577.899884097: ☑️ agreed by qiancai.
• 2024-08-23 08:59:59.856170243 +0000 UTC m=+515194.990620362: ☑️ agreed by Oreoxmt.

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #18467.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #18469.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.3: #18471.