Simplify HashMap declaration in test fixture

Co-authored-by: kuanghc1 <[email protected]>

Closes gh-27900

Author: springmonster

spring-aop/src/testFixtures/java/org/springframework/aop/testfixture/advice/MethodCounter.java

@@ -19,6 +19,7 @@
 import java.io.Serializable;
 import java.lang.reflect.Method;
 import java.util.HashMap;
+import java.util.Map;
 
 /**
  * Abstract superclass for counting advices etc.
@@ -31,7 +32,7 @@
 public class MethodCounter implements Serializable {
 
 	/** Method name --> count, does not understand overloading */
-	private HashMap<String, Integer> map = new HashMap<>();
+	private Map<String, Integer> map = new HashMap<>();
 
 	private int allCount;