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/functrace.cpp

@@ -35,7 +35,7 @@ static void handle_sigint(int)
     g_debug_session = nullptr;
 }
 
-static void print_function_call(String function_name, size_t depth)
+static void print_function_call(DeprecatedString function_name, size_t depth)
 {
     for (size_t i = 0; i < depth; ++i) {
         out("  ");