Delete the abandonment email task (#28998) (#29157)

* Add migration for deleting the abandonment email task

* Start+stop scheduler around the migration

* Update clj-kondo

* Rename migration

* Change to 46

* Fix define-migration

* Add test

* Update ns description

* Remove stop-scheduler! from migration

* Remove migration dependency on metabase.task

* Restore mdb.connection -> mdb

* Update custom-migrations ns docstring

* Remove unused code

* Fix comment

Co-authored-by: Cal Herries <39073188+calherries@users.noreply.github.com>