Reinstate support for Debian Buster

[apyrgio]

Dangerzone originally supported Debian Buster, but support got dropped once it switched to Podman on Linux (d50b0c1), which is not available on Debian Buster.

We have previously fixed this issue for Ubuntu Focal (#206) by relying to external repos for installing Podman. We can do the same for Debian Buster, and we have a WIP branch for that (support-buster-kudu). However, the installation fails with:

The following packages have unmet dependencies:
 podman : Depends: libseccomp2 (>= 2.4.3-1) but 2.3.3-4 is to be installed
          Recommends: slirp4netns (>= 100:1.1.8-3) but it is not going to be installed
          Recommends: containernetworking-plugins (>= 100:1.0.0-1) but it is not going to be installed
          Recommends: uidmap but it is not going to be installed
          Recommends: fuse-overlayfs but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

(see also this failing build)

So, besides adding an external repo, we need to handle the libseccomp2 incompatibility.

[apyrgio]

I propose not including this in the 0.4.0 release, as the schedule is tight.

[deeplow]

Debian buster is already EOL from the security standpoint. So I'll close the issue.