kbuild: deb-pkg: split debian/copyright from the mkdebian script

Copy debian/copyright instead of generating it by the 'cat' command.

I also updated '2018' to '2023' while I was here.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>