Make properties field final

Closes gh-30615

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/export/properties/PropertiesConfigAdapter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2022 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.
@@ -31,7 +31,7 @@ import org.springframework.util.Assert;
  */
 public class PropertiesConfigAdapter<T> {
 
-	private T properties;
+	private final T properties;
 
 	/**
 	 * Create a new {@link PropertiesConfigAdapter} instance.