raise postifx main.cf compatibility_level to 2

raise postifx main.cf compatibility_level to 2

otherwise mail received via TCP (localhost port 25) is not accepted, since
there is no default smptd_relay_restriction set, which would make the system an
open relay.

in compatibility_level 2 the default is sensibly set to:
'permit_mynetworks, permit_sasl_authenticated, defer_unauth_destination'

reported via the pve-user list:

https://pve.proxmox.com/pipermail/pve-user/2020-February/171368.html

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>