migrate-from-mariadb: add note about utf8mb4_uca1400_ai_ci

[dveeden]

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[gemini-code-assist]

Summary of Changes

Hello @dveeden, 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 enhances the TiDB documentation for users migrating from MariaDB by addressing a specific compatibility issue related to character set collations. It provides essential guidance on how to handle the utf8mb4_uca1400_ai_ci collation, which is not supported in TiDB, ensuring a smoother migration process.

Highlights

• Documentation Update: Added a crucial note to the MariaDB migration guide, informing users about TiDB's lack of support for the utf8mb4_uca1400_ai_ci collation, which is now the default in MariaDB. The documentation now recommends replacing it with utf8mb4_0900_ai_ci and clarifies the difference in UCA standard versions.

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[gemini-code-assist]

Code Review

This pull request adds a helpful note about the utf8mb4_uca1400_ai_ci collation not being supported in TiDB. I've suggested a few minor wording changes to align with the documentation style guide, focusing on clarity and avoiding contractions and passive voice. Overall, this is a good addition to the documentation.

[dveeden]

@hfxsd should we link to charset docs?

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-12-02 12:08:21.405532841 +0000 UTC m=+351646.219310423: ☑️ agreed by hfxsd.
• 2025-12-02 12:08:23.199302674 +0000 UTC m=+351648.013080246: ☑️ agreed by hfxsd.

[hfxsd]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hfxsd

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #22139.