Currently, there was no markdown page for deactivate-own-user API
endpoint. Created deactivate-own-user.md for the API page and
created a new owner client in test-api to reactivate the client
deactivated during testing.
Also changed endpoint name from deactivate-my-account to
deactivate-own-user, for better consistency with other endpoints.
Fixes#16163.
Suyash Vardhan Mathur