resource/gitlab_user: Add support for `deactivated` user state. Closes #897

This change will fix a regression from v3.9.0 to v3.10.0, see report in #897 (closed).

However, the regression is just because the user state has been changed outside of terraform, while it was supposed to be owner by terraform. It was changed outside of terraform, because the gitlab_user resource didn't support deactivating users.

Therefore, we could argue that this is actually a new feature and not a bug fix. But whatever, @ilpianista how urgent is the upgrade to 3.10 for you?