feat: Asynchronous initialization of Parse Server (#8232)

BREAKING CHANGE: This release introduces the asynchronous initialization of Parse Server to prevent mounting Parse Server before being ready to receive request; it changes how Parse Server is imported, initialized and started; it also removes the callback `serverStartComplete`; see the [Parse Server 6 migration guide](https://github.com/parse-community/parse-server/blob/alpha/6.0.0.md) for more details (#8232)

spec/ParseServer.spec.js

@@ -92,7 +92,7 @@ describe('Server Url Checks', () => {
         close = true;
       },
     });
-    const parseServer = ParseServer.start(newConfiguration);
+    const parseServer = ParseServer.startApp(newConfiguration);
   });
 
   it('does not have unhandled promise rejection in the case of load error', done => {