update serverless tidb_gc_life_time restriction (#19511)

[ti-chi-bot]

This is an automated cherry-pick of #19511

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

Serverless has now supported letting user specify gc life time, so previous description regarding this limit need to be modified.

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

It only applys to Serverless, so master should be enough.

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For details, see tips for choosing the affected versions.

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)

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Do your changes match any of the following descriptions?

• Delete files
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• Might cause conflicts after applied to another branch

[qiancai]

/approve

[ti-chi-bot]

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