chore: remove v5.4 from PR template

[qiancai]

What is changed, added or deleted? (Required)

• remove the v5.4 checkbox from .github/pull_request_template.md
• update README.md to mark release-5.4 documentation as archived and no longer updated

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)
• v6.1 (TiDB 6.1 versions)

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

• This PR is translated from:
• Other reference link(s):
  • https://github.com/pingcap/docs-cn/pull/18497/files
  • main: archive v5.4 docs docs-staging#133
  • archive: add TiDB v5.4 to the doc list docs-staging#134

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]

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