backend: automatically restart services in %post
    
    Fix #2141
    
    I asked on `#fedora-devel` about restarting a target and the response
    was "What would restarting target even mean?". I also didn't find any
    mentions of restarting targets in the documentation.
    
    The `man systemd.target` doesn't go into detail about the possible
    use-cases of targets, it only mentions "grouping units and as
    well-known synchronization points during start-up."
    
    ArchWiki even equates targets to SysV runlevels,
    https://wiki.archlinux.org/title/systemd#Targets
    so in this context, restarting targets doesn't make sense as well.
    
    I don't know if our `copr-backend.target` is a valid use-case but it
    IMHO doesn't matter. We can file a RFE for supporting targets in
    `%systemd_postun_with_restart` but for the time being, I would just
    change our specfile and list all services that should be restarted.