LibWeb: Port CSSKeyframesRules interface from DeprecatedString to String

Author: https://github.com/shannonbooth
Commit: 15334a68df
Pull-request: https://github.com/SerenityOS/serenity/pull/20799
Reviewed-by: https://github.com/ADKaster ✅

Userland/Libraries/LibWeb/CSS/CSSKeyframesRule.idl

@@ -1,6 +1,6 @@
 #import <CSS/CSSRule.idl>
 
-[Exposed=Window]
+[Exposed=Window, UseNewAKString]
 interface CSSKeyframesRule : CSSRule {
     attribute CSSOMString name;
     readonly attribute unsigned long length;