chore: remove v6.1 from PR template

[qiancai]

What is changed, added or deleted? (Required)

Remove the v6.1 checkbox from the PR template and mark release-6.1 as archived in README.md.

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

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 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)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[gemini-code-assist]

Code Review

This pull request removes TiDB v6.1 from the pull request template's list of affected versions and updates the README to mark the release-6.1 documentation as archived and no longer updated. There are no review comments, so I have no feedback to provide.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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Details

Needs approval from an approver in each of these files:

• OWNERS [qiancai]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #23247.
But this PR has conflicts, please resolve them!