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.cpp

@@ -17,7 +17,7 @@ Block::Block(Token token, Vector<ComponentValue>&& values)
 
 Block::~Block() = default;
 
-String Block::to_string() const
+DeprecatedString Block::to_string() const
 {
     StringBuilder builder;