Addressing: https://gitlab.com/gitlab-org/gitlab/-/issues/429803
Changelog: fixed

Changelog: added

As part of column removal lets first remove foreign key.

Since `application_settings` is part of `gitlab_main_cluster`
reduce foregin keys to `gitlab_main`.

The return value of table_schemas call can now contain 'nil' values

Unfortunately we cannot run tests from this script as it holds on to
database connections. So we remove automatic test running, and let
developer run tests themselves

This was something we required for en-masse creation but we do not need
to maintain this going forward

Changelog: other

This adds a helper script to automate loose foreign key swap.
Script does:

- create branch
- creates templated migration
- runs migrations to commit `db/structure.sql`
- updates YMLs for LFK
- updates specs for LFK
- generates LFK spec in a model
- commits
- pushes
- provides MR template