Automatically swap to closed registrations if an admin hasn't logged in recently

**Description:**

Synapse installs that have open registration should automatically swap to closed registration if an admin hasn't logged in recently, as a way to mitigate unmaintained or abandoned homeservers as a vector for abuse.

Inspired by the [latest release of Mastodon](https://github.com/mastodon/mastodon/releases/tag/v4.2.8) which includes two changes to address spam and abuse: one change is something that Synapse already does, which is defaulting to closed registrations, and the other is automatically swapping to closed registrations if an admin hasn't logged in recently (8 weeks).