LibWeb: Port MutationObserver interface from DeprecatedString to String

Author: https://github.com/shannonbooth
Commit: 0ce5a920f7
Pull-request: https://github.com/SerenityOS/serenity/pull/20968

Userland/Libraries/LibWeb/DOM/MutationObserver.idl

@@ -1,7 +1,7 @@
 #import <DOM/MutationRecord.idl>
 #import <DOM/Node.idl>
 
-[Exposed=Window, UseDeprecatedAKString]
+[Exposed=Window]
 interface MutationObserver {
 
     constructor(MutationCallback callback);