workstation/silverblue: Add third-party repositories
    
    Make fedora-release-workstation and fedora-release-silverblue
    Requires(meta): the fedora-workstation-repositories and
    fedora-flathub-remote packages that provide our recommended
    third-party repositories, so that they get installed on upgrades as well
    as new installs.
    
    (The repositories are disabled in packaging and only enabled by the
    fedora-third-party tool if the system is opted in.)
    
    https://fedoraproject.org/wiki/Changes/Third_Party_Software_Mechanism
    https://fedoraproject.org/wiki/Changes/Filtered_Flathub_Applications
    
    Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>