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/Utilities/env.cpp

@@ -17,7 +17,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
     bool ignore_env = false;
     StringView split_string {};
-    Vector<String> values;
+    Vector<DeprecatedString> values;
 
     Core::ArgsParser args_parser;
     args_parser.set_stop_on_first_non_option(true);