Use the non-deprecated exportRoomKeys method on CryptoApi (#12231)

* Expand the export test to check encryptMegolmKeyFile was called

* Use the non-deprecated exportRoomKeys method on CryptoApi

src/async-components/views/dialogs/security/ExportE2eKeysDialog.tsx

@@ -109,7 +109,7 @@ export default class ExportE2eKeysDialog extends React.Component<IProps, IState>
         // asynchronous ones.
         Promise.resolve()
             .then(() => {
-                return this.props.matrixClient.exportRoomKeys();
+                return this.props.matrixClient.getCrypto()!.exportRoomKeys();
             })
             .then((k) => {
                 return MegolmExportEncryption.encryptMegolmKeyFile(JSON.stringify(k), passphrase);