Modal: remove support for onFinished callback (#29852)

* Fix up type for `finished` result of Modal

The `finished` promise can be called with an empty array, for example if the
dialog is closed by a background click. This was not correctly represented in
the typing. Fix that, and add some documentation while we're at it.

* Type fixes to onFinished callbacks from Modal

These can all be called with zero arguments, despite what the type annotations
may say, so mark them accordingly.

* Remove uses of Modal `onFinished` property

... because it is confusing.

Instead, use the `finished` promise returned by `createDialog`.

* Modal: remove support for now-unused `onFinished` prop

* StopGapWidgetDriver: use `await` instead of promise chaining

* Fix up unit tests

test/unit-tests/components/views/settings/devices/deleteDevices-test.tsx

@@ -35,7 +35,7 @@ describe("deleteDevices()", () => {
         await deleteDevicesWithInteractiveAuth(mockClient, deviceIds, onFinished);
 
         expect(mockClient.deleteMultipleDevices).toHaveBeenCalledWith(deviceIds, undefined);
-        expect(onFinished).toHaveBeenCalledWith(true, undefined);
+        expect(onFinished).toHaveBeenCalledWith(true);
 
         // didnt open modal
         expect(modalSpy).not.toHaveBeenCalled();