Begin structural fixes and renamings on BBM update page

What does this MR do?

As a follow-up to !124821 (comment 1452030253) (thanks @anton) let's start tidying up https://docs.gitlab.com/ee/update/background_migrations.html by doing some structural work:

• Rename installation types, follow established precedent from Rename Omnibus and Source options in the docs (&10716)
• Pretty-print SQL queries for legibility
• Shift installation-specific instructions into tab format
• Add warning boxes to content I know from previous MRs to be "don't unless you REALLY know what you're doing"
• I deleted one item that was for 13.x, as that version is no longer supported.

This merge request is structural only. Lots of line edits are needed, but I want to do them after this MR merges.

@alexives and @rogerwoo will want to know I'm getting started on this work, but this MR needs no specific action from them. The follow-up MRs with line edits, however, might need their eyes.

Related issues

• Related to Rename Omnibus and Source options in the docs (&10716)
• Related to Retry failed batched background migrations (!124821 - merged) where I realized more cleanup is needed

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

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

Reviewer's 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 you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• 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.
  • Ensure a release milestone is set.
  • 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 reviews above. Maintainer's review can occur before or after a technical writer review.