Make invite linking to user case insensitive for private_commit_emails
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The following discussion from !151796 should be addressed:
- [ ] @dstull started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/151796#note_1907743452):
> **note (non-blocking):**
>
> We should follow-up here on how to handle case insensitivity here for private_commit_emails corner case.
issue