Rename UpdateUser to focus on phone attributes

[zachmargolis]

• Inlines most use cases for UpdateUser
• Renames UpdateUser to UpdateUserPhoneConfiguration because that's where the bulk of the logic is

Possible future work: refactor UpdateUserPhoneConfiguration interface to be more specific about what side effects, such as having something like:

UpdateUserPhoneConfiguration.new(user).update_defaults(
  phone_id: ...
  otp_make_default_number: ...
)