Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Utilities/uname.cpp

@@ -54,6 +54,6 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         parts.append(uts.machine);
     StringBuilder builder;
     builder.join(' ', parts);
-    puts(builder.to_string().characters());
+    puts(builder.to_deprecated_string().characters());
     return 0;
 }