Add username check to doctor (#20140) (#20671)

Add username check to doctor (#20140) (#20671)

* Add username check to doctor

- Add a new breaking change detector to Gitea's doctor, which checks if
all users still have a valid username according to Gitea. Given from
time-to-time we need to make changes, either due to new routes or due to
security, it's for a instance's admin to check if all users still have a
valid username.

* Fix extra argument

* Apply suggestions from code review

* Apply suggestions from code review