Base JWT service endpoint choice directly on the MatrixRTC mode

Since we removed the extra 'legacy' mode, meaning there's nothing simplified anymore by translating things over to a JWT service version.

(Also, simplify some things by making the mode static for the duration of a call.)

src/utils/test-viewmodel.ts

@@ -174,7 +174,7 @@ export function getBasicCallViewModelEnvironment(
           setE2EEEnabled: async () => Promise.resolve(),
         }),
       connectionState$: constant(ConnectionState.Connected),
-      matrixRTCMode$: constant(MatrixRTCMode.Compatibility),
+      matrixRTCMode: MatrixRTCMode.Compatibility,
       ...callViewModelOptions,
     },
     handRaisedSubject$,