Rename `gl_user_id` to `gitlab_user_id` in importer classes

What does this MR do?

Renames gl_user_id method in code importer classes to gitlab_user_id

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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #21772 (closed)