[9.18] chg: ci: Update issue closing regex in dangerfile.py

Update issue regex in danger file

The regular expression in `dangerfile.py` has been updated to match
the one in GitLab and bind9-qa (isc-projects/bind9-qa!41), i.e.
https://docs.gitlab.com/user/project/issues/managing_issues/#default-closing-pattern.

Backport of MR !10361

Merge branch 'backport-andoni/update-issue-regex-in-danger-file-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10364