Make MongoSessionUtils package private

See gh-2170

spring-session-data-mongodb/src/main/java/org/springframework/session/data/mongo/MongoSessionUtils.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017 the original author or authors.
+ * Copyright 2014-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,7 +27,7 @@ import org.springframework.lang.Nullable;
  *
  * @author Greg Turnquist
  */
-public final class MongoSessionUtils {
+final class MongoSessionUtils {
 
 	private MongoSessionUtils() {
 	}