contrib: debian: switch to dh-sequence-dkms

Follows b191f9a13d3005621ead9a727b811892264505ef from Debian's
packaging team at:
https://salsa.debian.org/zfsonlinux-team/zfs/

The previous build-dependency is kept as option, to still be able to
build on older Debian based distros (e.g. Ubuntu 20.04).

Without this building on Debian 12/bookworm does not work, as dkms
is a virtual package.

Reviewed-by: Umer Saleem <usaleem@ixsystems.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Closes #15304