[9.18] new: ci: Extend the prepare-release-announcement job to post release links

The prepare-release-announcement job is now extended so that after
creating the announcement MR, it posts a message with links to the newly
released versions to Mattermost.

Backport of MR !11860

Merge branch 'backport-andoni/extend-prepare-release-announcement-with-urls-message-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11870