system-variables: update the default value of datadir (#13336)

[ti-chi-bot]

This is an automated cherry-pick of #13336

What is changed, added or deleted? (Required)

as tittle

Which TiDB version(s) do your changes apply to? (Required)

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For details, see tips for choosing the affected versions.

• master (the latest development version)
• v7.1 (TiDB 7.1 versions)
• v7.0 (TiDB 7.0 versions)
• v6.6 (TiDB 6.6 versions)
• v6.5 (TiDB 6.5 versions)
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• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):system-variables: update description of datadir #13303

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hfxsd

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[hfxsd]

/merge

[ti-chi-bot]

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

Commit hash: 89534ab