tidbcloud: Update FLASHBACK CLUSTER TO warning for instance tiers (#23149)

[ti-chi-bot]

This is an automated cherry-pick of #23149

Corrected the warning regarding the applicability of the FLASHBACK CLUSTER TO syntax to include premium instances.

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[gemini-code-assist]

Code Review

This pull request updates the documentation to include the premium instance tier alongside starter and essential tiers for limitations regarding the FLASHBACK CLUSTER syntax and read-only system variables. The reviewer suggests using template variables instead of hardcoding the instance tier names in sql-statement-flashback-cluster.md to maintain consistency and support dynamic replacement.

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[qiancai]

/approve

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