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/EventHandler.cpp

@@ -9,7 +9,7 @@
 
 namespace Web::HTML {
 
-EventHandler::EventHandler(String s)
+EventHandler::EventHandler(DeprecatedString s)
     : value(move(s))
 {
 }