- make `UserActivity.InGame` and derive that to `InSoloGame` and `InMultiplayerGame`
- rename `SoloGame` to `InSoloGame`
- rename `MultiplayerGame` to `InMultiplayerGame`
Regressed in https://github.com/ppy/osu/pull/12204.
Adding this back in a central location for now, as each of the remaining cases will
need a local solution.