Use nav for new room list and label sections (#30134)

* Use nav for new room list and label sections

The old room list had a nav element but it was missed in the new one,
so add it and albel the sections. Also remove the test ID and use
this instead.

* Update snapshots

* Use the function we define above

test/unit-tests/components/structures/__snapshots__/RoomView-test.tsx.snap

@@ -403,6 +403,7 @@ exports[`RoomView for a local room in state ERROR should match the snapshot 1`]
       </div>
     </header>
     <main
+      aria-label="Room content"
       class="mx_RoomView_body"
     >
       <div
@@ -703,6 +704,7 @@ exports[`RoomView for a local room in state NEW should match the snapshot 1`] =
       </div>
     </header>
     <main
+      aria-label="Room content"
       class="mx_RoomView_body"
     >
       <div
@@ -1080,6 +1082,7 @@ exports[`RoomView for a local room in state NEW that is encrypted should match t
       </div>
     </header>
     <main
+      aria-label="Room content"
       class="mx_RoomView_body"
     >
       <div