Merge branch '507274-fix-nuget-symbols-unique-index-errors' into 'master'

Replace DB unique index with model validator

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175167



Merged-by: David Fernandez <dfernandez@gitlab.com>
Approved-by: Qingyu Zhao <qzhao@gitlab.com>
Approved-by: Dylan Griffith <dyl.griffith@gmail.com>
Approved-by: David Fernandez <dfernandez@gitlab.com>
Reviewed-by: Vitali Tatarintev <vtatarintev@gitlab.com>
Reviewed-by: David Fernandez <dfernandez@gitlab.com>
Reviewed-by: Moaz Khalifa <mkhalifa@gitlab.com>
Co-authored-by: moaz-khalifa <mkhalifa@gitlab.com>