LibWeb: Port CSSKeyframeRules interface from DeprecatedString to String

Userland/Libraries/LibWeb/CSS/CSSKeyframeRule.h

@@ -29,12 +29,12 @@ public:
     CSS::Percentage key() const { return m_key; }
     JS::NonnullGCPtr<CSSStyleDeclaration> style() const { return m_declarations; }
 
-    DeprecatedString key_text() const
+    String key_text() const
     {
-        return m_key.to_string().to_deprecated_string();
+        return m_key.to_string();
     }
 
-    void set_key_text(DeprecatedString const& key_text)
+    void set_key_text(String const& key_text)
     {
         dbgln("FIXME: CSSKeyframeRule::set_key_text is not implemented: {}", key_text);
     }

Userland/Libraries/LibWeb/CSS/CSSKeyframeRule.idl

@@ -1,6 +1,6 @@
 #import <CSS/CSSRule.idl>
 
-[Exposed=Window]
+[Exposed=Window, UseNewAKString]
 interface CSSKeyframeRule : CSSRule {
     attribute CSSOMString keyText;
     [SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;