add config unit for tidb_adaptive_closest_read_threshold

[glorv]

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hfxsd
• qiancai

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[glorv]

/cc @qiancai

[qiancai]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 4a81586

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #13856.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #13857.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.0: #13858.