Merge branch 'fix-desired-sharding-key-spec-for-migrations-in-progress' into 'master'

Use IF NOT EXISTS when adding desired sharding key test column

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/144942



Merged-by: Dylan Griffith <dyl.griffith@gmail.com>
Approved-by: Dylan Griffith <dyl.griffith@gmail.com>
Co-authored-by: Tiger <twatson@gitlab.com>