Fix multiplayer/playlists lounge screen disposing rooms synchronously

[frenzibyte]

• Closes Switching rulesets on the multiplayer screen sometimes causes a huge lag spike #26893

This starts to become noticeable as players began to create more rooms and playlists in osu!lazer. When changing the ruleset, RoomManager removes the previously existing rooms, signaling RoomsContainer to remove and dispose all existing rooms synchronously. This gets bad very quickly when the number of rooms exceeds about 10.

Before:

CleanShot.2024-02-04.at.17.13.44.mp4

After:

CleanShot.2024-02-04.at.17.15.17.mp4

(the delay between the loading layer hiding and the rooms actually appearing is an issue that occurs on master as well. not entirely sure what's going on there.)

[bdach]

Screen needs pooling and more DLUW at some point, but for now this'll do.