fix: ci: Escape literal dots in branch-name match regexes

The backports and merged-metadata rules used unescaped dots in their
branch-name regexes, causing them to over-match. Escape the dots so the
patterns match the intended version branch names exactly.

Assisted-by: Claude:claude-opus-4-8
Merge branch 'mnowak/ci-fix-regex-escaping' into 'main'

See merge request isc-projects/bind9!12172