[9.20] new: ci: Set up automatic rebasing for security-* branches

[9.20] new: ci: Set up automatic rebasing for security-* branches

Introduce a set of private branches containing only security fixes that
are automatically rebased onto the corresponding open source branches
whenever new changes are merged.  Each rebase triggers a basic build,
failing the CI job if the build breaks.

When a security-* branch is rebased, create a CI pipeline for its new
revision and rebase its corresponding bind-9.x-sub branch (if it exists)
on top of it, creating a rebase chain.

Report any failures in the process via Mattermost.

These changes enable treating security fixes similarly to other code
changes, without deferring merges all the way until release prep.

Backport of MR !11930

Merge branch 'backport-michal/autorebase-chain-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11931