Merge branch '517248-add-mr_migration_exclusions-table' into 'master'

Add excluded_merge_requests table

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182488



Merged-by: Tiger Watson <twatson@gitlab.com>
Approved-by: Ethan Urie <eurie@gitlab.com>
Approved-by: Tiger Watson <twatson@gitlab.com>
Reviewed-by: Ethan Urie <eurie@gitlab.com>
Co-authored-by: Patrick Bajao <ebajao@gitlab.com>
Co-authored-by: Kerri Miller <kerrizor@kerrizor.com>