Update copyright notices to 2021

debian/copyright

@@ -10,7 +10,7 @@ Files:
  debian/*
 Copyright:
  2005-2015 David Martínez Moreno <ender@debian.org>
- 2016 Christoph Biedl <debian.axhn@manchmal.in-ulm.de>
+ 2016-2021 Christoph Biedl <debian.axhn@manchmal.in-ulm.de>
 License: GPL-2
 
 License: GPL-2