partitioned-table: global index supports non-unique index

[Defined2014]

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• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
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What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): ddl: supports non-unique global index tidb#58678

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

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-06 06:08:43.447301347 +0000 UTC m=+338394.979713339: ☑️ agreed by qiancai.
• 2025-03-10 06:49:51.74636449 +0000 UTC m=+165747.351893417: ☑️ agreed by Oreoxmt.

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #20982.