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)

src/Options/Definitions.js

@@ -466,10 +466,6 @@ module.exports.ParseServerOptions = {
     env: 'PARSE_SERVER_SERVER_CLOSE_COMPLETE',
     help: 'Callback when server has closed',
   },
-  serverStartComplete: {
-    env: 'PARSE_SERVER_SERVER_START_COMPLETE',
-    help: 'Callback when server has started',
-  },
   serverURL: {
     env: 'PARSE_SERVER_URL',
     help: 'URL to your parse server with http:// or https://.',