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/CSS/CSSRule.cpp

@@ -24,7 +24,7 @@ void CSSRule::visit_edges(Cell::Visitor& visitor)
 }
 
 // https://www.w3.org/TR/cssom/#dom-cssrule-csstext
-String CSSRule::css_text() const
+DeprecatedString CSSRule::css_text() const
 {
     // The cssText attribute must return a serialization of the CSS rule.
     return serialized();