[18.11] Update gitlab-markup to 2.2.0 and rdoc to 8.1.0

What does this MR do and why?

Backport of Update gitlab-markup to 2.2.0 and rdoc to 8.1.0 (!262562 - merged).

Per !262719 (merged), we get rbs 4.2.0, and drop psych and stringio.

Differently to that, we drop solargraph entirely:

• rdoc 8 needs rbs >= 4
• Every solargraph that allows rbs 4 (0.59+) needs rubocop ~> 1.76
• GitLab 18.11's gitlab-styles 13.1.0 pins rubocop at 1.17.1

Solargraph is only used in dev; bumping gitlab-styles and rubocop would probably result in a bunch of new violations to exempt to get past CI (plus whatever fallout from the styles bump), and it doesn't seem worth it.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
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  • If this MR fixes a bug that affects customers, the customer label has been applied.
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• Ensure the E2E Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

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If you have questions about the patch release process, please:

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