Change diagnostics of foris -> reforis

Running diagnostics on TOS 6.0+ for Foris diagnostics module will output this warning:

$ /usr/share/diagnostics/diagnostics.sh foris
[...]
!! Program 'foris' not found !!

See: https://gitlab.nic.cz/turris/diagnostics/-/blob/master/modules/72_foris.module#L23-27

check_installed foris && {
    section "Foris Lighttpd configuration segment"
    /usr/share/foris/lighttpd-dynamic-conf config
}

We should drop detection of Foris and check for reForis and it's config instead.