Rename users with namespace ending with .git

What does this MR do?

Rename users with namespace ending with .git to _git.

Are there points in the code the reviewer needs to double check?

migration

Why was this MR needed?

Because .git in not allowed as username suffix. Fix 500 error when existing user ends with .git.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/25246