Use .well-known to discover a default rendezvous server for use with Sign in with QR (#11655)

test/components/views/settings/devices/LoginWithQR-test.tsx

@@ -51,6 +51,7 @@ function makeClient() {
         currentState: {
             on: jest.fn(),
         },
+        getClientWellKnown: jest.fn().mockReturnValue({}),
     } as unknown as MatrixClient);
 }