Update backport docs for maintained versions and cherry-picks

What does this MR do?

Updates doc/development/documentation/backporting.md with the two points left open after the earlier clarification MR:

• Any maintainer can backport to the maintained versions, not only the current stable release. The release documentation describes this as a soft launch, so this page now points to the maintenance policy's definition of maintained versions.
• The backport merge request steps now link to the release documentation instead of restating them, and say to cherry-pick so the same commit exists on both branches.

The section for older releases is reworded so it means releases older than the maintained versions.

Related issues

• Resolves #584457
• Review notes this follows up on: !213983 (comment 2911337737) and !213983 (comment 2911643508)

Notes for reviewers

The author of the earlier MR chose to keep the page focused on the current stable branch only. Please confirm that the wider "maintained versions" wording is what you want before merging. The heading changed, so check for inbound anchor links to the old heading.

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• No new page is added, so no product availability details are needed.

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 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.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.