pve6to7: use new flags API

the old one is not available post-upgrade, let's use a single codepath
for this.

the new API only allows querying user-settable flags, but the only flags
we check besides 'noout' are not relevant for an upgrade of PVE 6.x to
7.x (PVE 6.x only supports Nautilus+ which requires these flags to be
set in order to work) so we can just drop those outdated checks instead
of extending/refactoring the API.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>