Update copyright years in c++tools

While looking at PR100731, I have noticed the copyright years are 2020-ish
only.  This patch adds it to update-copyright.py and updates those.

2021-05-25  Jakub Jelinek  <jakub@redhat.com>

contrib/
	* update-copyright.py: Add c++tools.
c++tools/
	* Makefile.in: Update copyright year.
	* configure.ac: Likewise.
	* resolver.cc: Likewise.
	* resolver.h: Likewise.
	* server.cc: Likewise.
	(print_version): Update copyright notice date.