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

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[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template's list of affected versions and updates the README to mark the release-6.1 documentation as archived and no longer updated.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.

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