update precision of DATETIME and TIMESTAMP values (#13969)

[ti-chi-bot]

This is an automated cherry-pick of #13969

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What is changed, added or deleted? (Required)

Fix the precision of DATETIME and TIMESTAMP values from milliseconds to microseconds.

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• v7.0 (TiDB 7.0 versions)
• v6.6 (TiDB 6.6 versions)
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[qiancai]

/merge

[ti-chi-bot]

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

Commit hash: a6a30ca