Clarify that %golang_arches_future are meant for RHEL 10
    
    Since the RHEL conditional was only targeting RHEL 9, it wasn't clear
    whether it needed updating for RHEL 10.  So, it's better to say that
    %golang_arches are for RHEL 9 and older, and %golang_arches_future are
    for Fedora and RHEL 10 onwards.
    
    This doesn't change any behaviour of the built artifacts, because the
    build is only shared with RHEL 9 onwards.  Hence, a conditional checking
    for RHEL 9 is the same as one checking for RHEL 9 and older.
    
    There's no need to do a build just for this.