Draft: Cleanup version histories and other tier related things

What does this MR do?

This MR will remove the tier from version histories and headings, where it is not needed.
Every file will be its own commit and the commit description states what was changed and why.

See !68694 (comment 659685527)

Open questions

• administration/operations/unicorn.md -> has reached the date of deletion !69669 (merged)
• administration/packages/index.md -> content of master is different to selfhosted 14.2.3 (RubyGems are still listed as "Accepting Contributions") !69670 (merged)
• api/graphql/custom_emoji.md -> Mentions multiple times, that the feature is not ready for production use. Version history however, recommends it for production use !69677 (merged)
• api/dora4_project_analytics.md -> page mentions these endpoints will be removed in 14.0. Should this page get deleted? !69684 (merged)
• api/feature_flag_specs.md -> page mentions the api has been deleted in 14.0. Should this page get deleted? !69686 (merged)
• api/feature_flags_legacy.md -> page mentions the api has been deleted in 14.0. Should this page get deleted? !69686 (merged)
• api/notes.md -> in the overview, epic notes are marked as premium, the actual heading is marked as ultimate !69692 (merged)
• api/notification_settings.md -> Epics are available at premium level, are the notifications ultimate only or is there an error? !69692 (merged)
• api/resource_label_events.md -> Epics are available at premium level, are the label events ultimate only or is there an error? !69696 (merged)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are only adding documentation, do not add any of the following labels:

• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.