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

src/components/structures/auth/SetupEncryptionBody.tsx

@@ -90,14 +90,15 @@ export default class SetupEncryptionBody extends React.Component<IProps, IState>
         // We need to call onFinished now to close this dialog, and
         // again later to signal that the verification is complete.
         this.props.onFinished();
-        Modal.createDialog(VerificationRequestDialog, {
+        const { finished: verificationFinished } = Modal.createDialog(VerificationRequestDialog, {
             verificationRequestPromise: requestPromise,
             member: cli.getUser(userId) ?? undefined,
-            onFinished: async (): Promise<void> => {
-                const request = await requestPromise;
-                request.cancel();
-                this.props.onFinished();
-            },
+        });
+
+        verificationFinished.then(async () => {
+            const request = await requestPromise;
+            request.cancel();
+            this.props.onFinished();
         });
     };