v7.5.1: tikv configuration doc: add gc thread number

[Oreoxmt]

What is changed, added or deleted? (Required)

v7.5.1 adds num-threads

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[ti-chi-bot]

@tonyxuqqi: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-22 03:31:54.125999224 +0000 UTC m=+500802.873622334: ☑️ agreed by hfxsd.

[Oreoxmt]

/unhold
/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch master: #16694.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #16695.