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/HTML/HTMLFormElement.cpp

@@ -152,7 +152,7 @@ void HTMLFormElement::remove_associated_element(Badge<FormAssociatedElement>, HT
 }
 
 // https://html.spec.whatwg.org/#dom-fs-action
-String HTMLFormElement::action() const
+DeprecatedString HTMLFormElement::action() const
 {
     auto value = attribute(HTML::AttributeNames::action);