Rename groups with .git in the end of the path

What does this MR do?

Renames group name that ends with .git to _git

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

migration

Why was this MR needed?

To prevent 404/500 errors for users who have such groups

Screenshots (if relevant)

no

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • 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?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/25862

Once deployed to GitLab.com this one https://gitlab.com/gitlab-com/support-forum/issues/1358