Registration Token requirement can be bypassed by logging in with SSO

This issue has been migrated from [#11067](https://github.com/matrix-org/synapse/issues/11067).

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### Description

Registration Token requirement (where you need a token given by an admin to sign up) can be bypassed by using a Third Party (OpenID) login service
I wish to have OpenID enabled on my private homeserver for the convenience of the people I give access to.
And I have registration tokens enabled since my Homeserver is private. (duh)

### Steps to reproduce

- Create account via Third Party (Twitch, Google, Github, Whatever) with Registration Token requirement enabled.

### Version information

- **Homeserver**: Would rather not provide publicly due to it being private, however it is self-hosted.

If not matrix.org:

- **Version**: 1.44.0

- **Install method**: Docker-Compose

- **Platform**: Ubuntu 20.04 Server running on actual hardware, Homeserver running in Docker Container

No source file or test is named. Start by reproducing registration on Synapse 1.44.0 with registration tokens enabled and a third-party login, then trace the registration and SSO flow; done means an SSO-based account cannot be created without a valid registration token.

Written by the indexing model from the issue text.