Add warning to "New auto-deploy branch" message when branch is behind master

What does this MR do and why?

Describe in detail what your merge request does and why.

Add a warning and ping release managers when there are commits in the default branch that have not been deployed for 7 days or more.

This MR modifies the Slack notification that is sent to the #releases channel when new auto deploy branches are created. The modification is behind a feature flag called notify_branch_too_far_behind.

With warning:

Without warning (this is the same as the current Slack notification):

gitlab-com/gl-infra/delivery#2350 (closed)

Author Check-list

• Has documentation been updated?