In order to veryfy post-deployment migrations add a new CI job that
executes migrations in two steps, similar to what happens during
zero-downtime deployments.

Related to
https://gitlab.com/gitlab-com/gl-infra/production/-/issues/7762#note_1106257962.

Add a new version of the db:check-migrations job that runs in a
decomposed pipeline, and tests migrations appropriately in that
environment.

Signed-off-by: Rémy Coutable <remy@rymai.me>

In the CI job that validates migration schema changes, the current
behavior did not account for multiple migrations in one MR, and so ran
the migrations based on the output of `git diff`.

This change fixes that, so all migrations in `post_migrate` will run
first in reverse timestamp order, and then migrations in `migrate` will
run in reverse timestamp order.

Add a new script that can be run in CI to verify that migrations added
in a merge request cleanly rollback to the previous known database state
on the target branch.