Fix count and update in MongoDb stores

* The `into()` query can't infer the type for entity if we provide `Object.class`
* The `updateFirst()` does not support sort queries any more - replace with `findAndModify()`
* Add `getMessageGroupCount()` into tests

**Cherry-pick to 5.3.x & 5.2.x**

spring-integration-mongodb/src/test/java/org/springframework/integration/mongodb/store/AbstractMongoDbMessageGroupStoreTests.java

@@ -95,6 +95,7 @@ public abstract class AbstractMongoDbMessageGroupStoreTests extends MongoDbAvail
 		assertThat(messageA.getHeaders().getId()).isEqualTo(retrievedMessage.getHeaders().getId());
 		// ensure that 'message_group' header that is only used internally is not propagated
 		assertThat(retrievedMessage.getHeaders().get("message_group")).isNull();
+		assertThat(store.getMessageGroupCount()).isEqualTo(1);
 	}
 
 	@Test