replace `Repo.get_by(User, ap_id: ap_id)` with `User.get_by_ap_id(ap_id)`

test/web/activity_pub/activity_pub_test.exs

test/web/mastodon_api/status_view_test.exs

test/web/salmon/salmon_test.exs

test/web/twitter_api/twitter_api_controller_test.exs