Rename users routing from /u/:username to /users/:username for consistency with other routes

What does this MR do?

Renames /u/:username to /users/:username

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

Why was this MR needed?

To follow consistency with other routes (like groups) and UsersController name. Now when you can use /:username for accessing user page there is no need in shortcut like /u/

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• 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?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23174