Bump copyright year

Author: horgh

README.md

@@ -437,6 +437,6 @@ The GeoIP2 PHP API uses [Semantic Versioning](https://semver.org/).
 
 ## Copyright and License ##
 
-This software is Copyright (c) 2013-2020 by MaxMind, Inc.
+This software is Copyright (c) 2013-2023 by MaxMind, Inc.
 
 This is free software, licensed under the Apache License, Version 2.0.