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/Parser/Block.h

@@ -32,7 +32,7 @@ public:
 
     Vector<ComponentValue> const& values() const { return m_values; }
 
-    String to_string() const;
+    DeprecatedString to_string() const;
 
 private:
     Block(Token, Vector<ComponentValue>&&);