tidb-lightning: add checking PITR and active changefeed task

[Frank945946]

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Update tidb-lightning-prechecks

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[lilin90]

/lgtm

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-07-14 10:45:25.468975932 +0000 UTC m=+2514978.192154915: ☑️ agreed by lilin90.

[lilin90]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #20654.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #20655.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #20656.