Merge branch '456823-create-index-on-user-assignments' into 'master'

Create index on user_add_on_assignments

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/150430



Merged-by: Jon Jenkins <jjenkins@gitlab.com>
Approved-by: Mohamed Hamda <mhamda@gitlab.com>
Approved-by: Jon Jenkins <jjenkins@gitlab.com>
Reviewed-by: Jon Jenkins <jjenkins@gitlab.com>
Reviewed-by: Mohamed Hamda <mhamda@gitlab.com>
Co-authored-by: Bishwa Hang Rai <bhrai@gitlab.com>