Readd index_on_users_name_lower if necessary

Stan Hurequested to merge
sh-readd-users-index-if-needed into master

What does this MR do and why?

This index was added in GitLab 10.5 with gitlab-foss!17158 (merged), but it was gated on a Gitlab::Database.postgresql? check. We've seen some older installations missing this index. Re-add it if is missing.

References

Relates to https://gitlab.com/gitlab-com/request-for-help/-/issues/3342

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Edited by Stan Hu

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