Update copyright notices to 2023

debian/copyright

@@ -9,7 +9,7 @@ License: GPL-3.0+
 Files:
  debian/*
 Copyright:
- 2020-2021 Christoph Biedl <debian.axhn@manchmal.in-ulm.de>
+ 2020-2023 Christoph Biedl <debian.axhn@manchmal.in-ulm.de>
 License: GPL-3.0+
 
 License: GPL-3.0+