chore: update release docs and remove legacy release scripts

[paulbalandan]

Description
This PR cleans up the maintainer release documentation and removes the legacy release scripts that were superseded by GitHub Actions.

• Remove the legacy release scripts (admin/release, release-config, release-deploy, release-userguide, release-revert, release-notes.bb) and admin/workflow.md, which documented that old process. They predate the current Actions-based deployment ("Deploy Distributable Repos") and contradict admin/RELEASE.md. Nothing else references them.
• admin/RELEASE.md:
  • Fix the documented NEXT_MINOR values to include the caret ('^4.y-dev', '^5.0-dev'), matching what prepare-release.php actually writes to admin/starter/builds.
  • Remove the incorrect "(if releasing the minor version)" qualifier: prepare-release.php updates version in conf.py unconditionally.
  • Convert the remaining plain bullets to * [ ] checkboxes so the whole file works as a copyable checklist.
  • Add a note to the "Manual User Guide Process" appendix clarifying it is only needed when the automatic deployment fails.
• admin/README.md: drop the "Release Building Scripts" section and other references to the deleted files, document prepare-release.php and create-new-changelog.php (previously undocumented), and fix the stale claim that CHANGELOG.md is generated by GitHub Changelog Generator.
• utils/check_permission_x.php: remove the deleted scripts from the executable-bit allowlist.

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