Remove the post-deploy migration jobs from the coordinated pipeline

As part of &585 (closed) post migrations were removed from the coordinated pipeline into an independent post-deploy migration (PDM) pipeline.

Exit criteria

• Remove the postdeploy-migrations:gstg and postdeploy-migration:gprd jobs on staging and production from the coordinated pipeline CI configuration
• Remove the notify_failure:gstg:postdeploy-migrations and the notify_failure:gprd:postdeploy-migrations from the coordinated pipeline CI configuration.
• Remove the "Handling post-migrations not being executed" doc bit added on gitlab-org/release/docs!480 (diffs)