AK+Everywhere: Rename String to DeprecatedString

We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)

Userland/Libraries/LibWeb/DOM/QualifiedName.cpp

@@ -67,7 +67,7 @@ void QualifiedName::Impl::make_internal_string()
         return;
     }
 
-    as_string = String::formatted("{}:{}", prefix, local_name);
+    as_string = DeprecatedString::formatted("{}:{}", prefix, local_name);
 }
 
 }