Merge branch '425974-drop-index_namespaces_on_created_at-for-gitlab-com' into 'master'

Drop an unused index for GitLab.com

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/134948



Merged-by: Prabakaran Murugesan <pmurugesan@gitlab.com>
Approved-by: Prabakaran Murugesan <pmurugesan@gitlab.com>
Reviewed-by: Krasimir Angelov <kangelov@gitlab.com>
Reviewed-by: Prabakaran Murugesan <pmurugesan@gitlab.com>
Co-authored-by: Krasimir Angelov <kangelov@gitlab.com>