[9.18] chg: ci: Mark merged security fixes as "Not released yet"

[9.18] chg: ci: Mark merged security fixes as "Not released yet"

Adjust the triggering rules for the "merged-metadata" CI job so that
merge requests merged into security-* branches are automatically
assigned to the "Not released yet" milestone, just like merge requests
targeting public branches.  This enables merge requests containing
security fixes to be correctly processed by release automation scripts.

Backport of MR !11984

Merge branch 'backport-pspacek/extend-not-released-yet-milestone-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11988