[paulbalandan]

Description
Rewrites admin/RELEASE.md so that any release manager can follow it without prior context.

• The 4.x.x, 4.y and 4.z notation is replaced by four named variables (VERSION, FEATURE_BRANCH, NEW_FEATURE_BRANCH, NEXT_VERSION). A table gives patch and minor examples and the old name of each.
• The checkbox list is now 21 numbered steps in five phases, with patch-only and minor-only steps marked.
• Every command is in its own code block and can be copied as it is. Pull requests are created with gh pr create, and the Prep pull request body keeps the repository's template.
• The Ready code pull request (develop into master) must be merged with a merge commit, never squashed. The guide says so in a callout and in step 14, with the reason. A table lists the merge method for each pull request.
• Gaps in the old guide are filled:
  • prepare-release.php starts from the local develop, so a step now updates it first.
  • Edits made before prepare-release.php must stay uncommitted, and only tracked files are committed.
  • After a minor release, the old feature branch no longer exists, so the post-release sync is skipped.
  • The stale "clone both repositories" step is removed, because the scripts need origin (the fork) and upstream.
• The edit history is a table with dates in UTC.
• The announcement step points to the forum-announcement skill.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide