Introduce 'value' alias for 'attribute' in @AliasFor

SPR-11512 introduced support for annotation attribute aliases via
@AliasFor, requiring the explicit declaration of the 'attribute'
attribute. However, for aliases within an annotation, this explicit
declaration is unnecessary.

This commit improves the readability of alias pairs declared within an
annotation by introducing a 'value' attribute in @AliasFor that is an
alias for the existing 'attribute' attribute. This allows annotations
such as @ContextConfiguration from the spring-test module to declare
aliases as follows.

public @interface ContextConfiguration {

     @AliasFor("locations")
     String[] value() default {};

     @AliasFor("value")
     String[] locations() default {};

    // ...
}

Issue: SPR-13289

src/asciidoc/whats-new.adoc

@@ -429,10 +429,10 @@ method has been added.
 ----
 public @interface ContextConfiguration {
 
-    @AliasFor(attribute = "locations")
+    @AliasFor("locations")
     String[] value() default {};
 
-    @AliasFor(attribute = "value")
+    @AliasFor("value")
     String[] locations() default {};
 
     // ...