Deprecate ConfigRetriever#getConfigAsFuture static method

[vietj]

In Vert.x 4 this method is now an instance method of ConfigRetriever, so we deprecate this in 3.x

// deprecated
Future<JsonObject> fut = ConfigRetriever. getConfigAsFuture(retriever);

// instead use in 3.X
fut = Future.future(retriever::getConfig);

// or in 4.0
fut = retriever.getConfig();