Have option to use the old room list

### Your use case

#### What would you like to do?

I'd like for there to be an option to enable the old room list view. Even if it's themed similar to the new one, I would like to see an option to use the old layout.

#### Why would you like to do it?

The new one is too difficult to navigate from an organization standpoint. The sudden switch is also very jarring, and the new buttons that are on the top ("Unreads", "People", "Rooms", etc.) don't really add much to the experience while taking up screen space. Specifically, "Unreads" and "Mentions" feel very pointless when you already see dots indicating unread messages or notifications next to a room name. The "Rooms" button is very pointless since you are already looking at a list of rooms. "People" is also just taking up screen real estate since I specifically want to keep DMs conceptually separate from a space. "Invites" is hidden by default for me unless I click a drop-down, but I would already see a much clearer notification for these in the old home or "#" symbol in the space list.

To continue, the drop-down menu next to the space name is much more sparse now, and I'm not 100% sure where to find many of the options I used to use there.

Basically, for someone who uses Matrix and Element in the way I do, this feels like a step backwards and hinders usability. None of these are issues in the old room list for a user such as myself, so an option to go back to it so I wouldn't have to stay on an out-of-date version of Element would be great.

#### How would you like to achieve it?

An option in the preferences would be perfect. Having the new room list enabled by default but then having to go into Labs to disable it would also be acceptable. I was using the Labs feature to disable the new rooms list up through 1.12.23, and was staying on older versions of Element before I knew about that option.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by locating the room-list and preferences or Labs implementations, then determine how an old-layout option would be selected and maintained; done means users can enable the old room list without staying on an outdated Element version.

Written by the indexing model from the issue text.