chore: replace Mergeable with Carson's welcome, template-enforcer, and no-response-closer

[paulbalandan]

Description
Final step of the move onto Carson, following #10529, #10547, and #10556. This PR retires the Mergeable app, which was the last external app handling our repository automation.

Mergeable did two things, and they map to three Carson subscribers:

• The greeting on a contributor's first PR becomes welcome. The message is ported verbatim, minus the "mergeable bot" signature.
• The bug report template check on new issues becomes template-enforcer, paired with no-response-closer for the closing.

Changes:

• .github/carson.yml: enable welcome, template-enforcer, and no-response-closer.
• .github/workflows/run-carson.yml: bump Carson to v1.6.0, add ready_for_review for the greeting on former drafts, and add a daily schedule at 00:00 UTC for the closer.
• Remove .github/mergeable.yml.

How the issue check behaves now:

• An issue missing ### PHP Version or ### CodeIgniter4 Version gets a comment listing what is missing and the needs template label.
• When the author edits the description to comply, Carson removes the label.
• An issue still carrying the label after 2 days without activity is closed as not planned, with a pointer to the bug report form and the forums. Any comment or edit resets the timer.
• Authors with the admin, maintain, write, or triage role skip the check. This replaces the hardcoded list of usernames, so it no longer needs editing when the team changes.

Behaviour changes:

• Mergeable closed a non-compliant issue immediately. The author now gets a grace period to fix the description instead of having to open a new issue.
• A first-time contributor is someone with no earlier PR in this repository, merged or not. Mergeable also re-greeted authors whose earlier PRs were never merged, on every new PR.
• A PR opened as a draft is greeted once, when it is marked ready for review.
• Only first-time PRs are greeted. First-time issues and returning contributors get no greeting, same as before.

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

[michalsn]

Sounds great!