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/Games/Snake/SnakeGame.cpp

@@ -250,7 +250,7 @@ void SnakeGame::game_over()
         text.append("\nThat's a new high score!"sv);
     }
     GUI::MessageBox::show(window(),
-        text.to_string(),
+        text.to_deprecated_string(),
         "Game Over"sv,
         GUI::MessageBox::Type::Information);