Raise MATE spin max size to 3GiB
    
    Similar to the Cinnamon case, the maintainer decided on this back
    around F31 time:
    https://bugzilla.redhat.com/show_bug.cgi?id=1756605
    and edited the wiki page:
    https://fedoraproject.org/w/index.php?title=Releases/31/Spins&diff=554050&oldid=554025
    but the change was lost in subsequent releases. As the max size
    is up to the maintainer's discretion (AIUI), we should keep this
    change.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>