deb: add support for building debian 11 "Bullseye"

[thaJeztah]

depends on:

• deb: make dh-systemd dependency optional as it's deprecated #520 deb: make dh-systemd dependency optional as it's deprecated
• Jenkinsfile: test building Debian 11 "bullseye" containerd-packaging#213 Jenkinsfile: test building Debian 11 "bullseye"
  • deb: make dh-systemd dependency optional as it's deprecated containerd-packaging#212 deb: make dh-systemd dependency optional as it's deprecated
  • deb: keep '/var/lib/apt/lists/' to allow building for Debian unstable containerd-packaging#219 deb: keep '/var/lib/apt/lists/' to allow building for Debian unstable

[thaJeztah]

Interesting failure (go modules?)

+ go build -buildmode=pie -ldflags=-linkmode=external -o /usr/local/bin/rootlesskit github.com/rootless-containers/rootlesskit/cmd/rootlesskit
go: github.com/insomniacslk/dhcp@v0.0.0-20210120172423-cc9239ac6294 requires
	github.com/mdlayher/ethernet@v0.0.0-20190606142754-0394541c37b7 requires
	golang.org/x/net@v0.0.0-20190603091049-60506f45cf65 requires
	golang.org/x/crypto@v0.0.0-20190308221718-c2843e01d9a2: invalid version: git fetch --unshallow -f origin in /go/pkg/mod/cache/vcs/f72f7a6f267d2fe79b1e174efab21a591b07c84ef53364f65b8281fda7000b49: exit status 128:
	fatal: git fetch-pack: expected shallow list
error: Bad exit status from /var/tmp/rpm-tmp.AKTGRN (%build)
    Bad exit status from /var/tmp/rpm-tmp.AKTGRN (%build)