Prevent compilation warnings with @Nullable

This commit replaces `@Nonnull(when = When.MAYBE)` meta-annotation in
`org.springframework.lang.Nullable` by `@CheckForNull` in order to
prevent `unknown enum constant When.MAYBE` compilation warnings.

Closes spring-projectsgh-27183

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -22,9 +22,8 @@
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
-import javax.annotation.Nonnull;
+import javax.annotation.CheckForNull;
 import javax.annotation.meta.TypeQualifierNickname;
-import javax.annotation.meta.When;
 
 /**
  * A common Spring annotation to declare that annotated elements can be {@code null}
@@ -50,7 +49,7 @@
 @Target({ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD})
 @Retention(RetentionPolicy.RUNTIME)
 @Documented
-@Nonnull(when = When.MAYBE)
+@CheckForNull
 @TypeQualifierNickname
 public @interface Nullable {
 }