Debian Bullseye EOL

The EOL of debian bullseye causes the packer build of repository to fail since the backports are not upstream anymore. I assume that this also effects other packer build which still reply on bullseye. Using bookworm solved it, I have not tried it with trixie yet, but bookworm is also EOL so I suggest to migrate to trixie.

The docs of repository should be updated as well since it claims that bullseye has to be used.