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/Libraries/LibKeyboard/CharacterMapFile.cpp

@@ -19,7 +19,7 @@ ErrorOr<CharacterMapData> CharacterMapFile::load_from_file(DeprecatedString cons
         full_path.append("/res/keymaps/"sv);
         full_path.append(filename);
         full_path.append(".json"sv);
-        path = full_path.to_string();
+        path = full_path.to_deprecated_string();
     }
 
     auto file = TRY(Core::File::open(path, Core::OpenMode::ReadOnly));