[9.18] new: ci: Prepare release announcement MR

In the 'release' stage, create an MR automatically with the
corresponding release announcement. The input for this is taken from
metadata.json in bind9-qa.

Backport of MR !11039

Merge branch 'backport-andoni/release-announcement-preparation-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11041