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/index.js

@@ -271,8 +271,6 @@ export interface ParseServerOptions {
   :ENV: PARSE_SERVER_PLAYGROUND_PATH
   :DEFAULT: /playground */
   playgroundPath: ?string;
-  /* Callback when server has started */
-  serverStartComplete: ?(error: ?Error) => void;
   /* Defined schema
   :ENV: PARSE_SERVER_SCHEMA
   */