Command to demote admin users in a room

Depends on https://github.com/element-hq/synapse/issues/16917

For example either making this use the new Admin API when required

```plaintext
!mjolnir powerlevel [room alias/ID]
```

Or a new command something along the lines of

```plaintext
!mjolnir remove admin [user alias/ID]
```

Start by reading the dependency issue 16917 and the existing handling for the shown !mjolnir powerlevel command. Decide whether to extend that command or add !mjolnir remove admin, then implement the selected path using the new Admin API when required. Done means the command can demote a specified user in the target room.

Written by the indexing model from the issue text.