Rely on default retention in `@Contract`

Closes gh-35027

spring-core/src/main/java/org/springframework/lang/Contract.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 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.
@@ -18,8 +18,6 @@ package org.springframework.lang;
 
 import java.lang.annotation.Documented;
 import java.lang.annotation.ElementType;
-import java.lang.annotation.Retention;
-import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
 /**
@@ -77,7 +75,6 @@ import java.lang.annotation.Target;
  * NullAway custom contract annotations</a>
  */
 @Documented
-@Retention(RetentionPolicy.CLASS)
 @Target(ElementType.METHOD)
 public @interface Contract {