Do not merge until v8.5.4: tidb: change the default value of tidb_mpp_store_fail_ttl (#21410)

[ti-chi-bot]

This is an automated cherry-pick of #21410

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• This PR is translated from: tidb: mark tidb_mpp_store_fail_ttl as deprecated docs-cn#20668
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[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the documentation for TiDB system variables to reflect a significant change in how TiDB handles newly started TiFlash nodes. It specifically addresses the deprecation of the tidb_mpp_store_fail_ttl variable, aligning the documentation with the improved system responsiveness where delays are no longer required.

Highlights

• Deprecation of tidb_mpp_store_fail_ttl: The system variable tidb_mpp_store_fail_ttl is now deprecated starting from v9.0.0. Its value is fixed to 0s, indicating that TiDB no longer waits before sending queries to newly started TiFlash nodes, as delays are no longer necessary to prevent query failures.
• Updated Default Value and Description: The default value for tidb_mpp_store_fail_ttl has been updated to 0s (from 60s in v8.5.3 and earlier versions), and the description clarifies the new behavior regarding TiFlash nodes.

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[gemini-code-assist]

Code Review

This pull request correctly marks the tidb_mpp_store_fail_ttl system variable as deprecated. The changes accurately reflect its new behavior. I've added a couple of suggestions to enhance the clarity and readability of the documentation.