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/tabs/user/EncryptionUserSettingsTab-test.tsx

@@ -62,7 +62,7 @@ describe("<EncryptionUserSettingsTab />", () => {
         );
         expect(asFragment()).toMatchSnapshot();
 
-        const spy = jest.spyOn(Modal, "createDialog").mockReturnValue({} as any);
+        const spy = jest.spyOn(Modal, "createDialog").mockReturnValue({ finished: new Promise(() => {}) } as any);
         await user.click(screen.getByText("Verify this device"));
         expect(spy).toHaveBeenCalled();
     });