Fix stale versioned docs links (#22875)

[ti-chi-bot]

This is an automated cherry-pick of #22875

What changed

This PR updates stale documentation links reported in #22867:

• Moves versioned TiDB v7.2, v7.6, and v8.3 links to docs-archive.pingcap.com, preserving the original versioned targets.
• Updates the TiDB Cloud Starter TLS link to the current secure-connections-to-serverless-clusters page.
• Removes the broken TiDB roadmap hyperlink from the TiDB Cloud roadmap page because the linked docs page no longer exists in the current docs site.

Verification

• rg -n https://docs\\.pingcap\\.com/tidb/v(7\\.2|7\\.6|8\\.3)/|https://docs\\.pingcap\\.com/tidbcloud/secure-connections-to-starter-clusters/|https://docs\\.pingcap\\.com/tidb/dev/tidb-roadmap -g *.md returns no matches.
• Checked each new external URL introduced in the diff with Invoke-WebRequest; all returned HTTP 200.
• git diff --check
• npx --yes markdownlint-cli@0.17.0 <changed files>

[gemini-code-assist]

Code Review

This pull request updates numerous stale documentation links, primarily migrating older versioned content to the archive domain and updating TiDB Cloud cluster terminology from "starter" to "serverless". It also simplifies content blocks in the execution plan documentation and removes the TiDB kernel roadmap section. However, the changes have introduced critical issues in the form of unresolved Git merge conflict markers in releases/release-8.5.0.md and tidb-cloud/sql-concepts.md. Feedback has been provided to resolve these conflicts and to ensure MySQL version notation aligns with the repository style guide.

[qiancai]

/approve

[ti-chi-bot]

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