fix #4859: properly configure TLSv1.3 only mode

fix #4859: properly configure TLSv1.3 only mode

set_min/max_proto_version is recommended upstream nowadays, and it seems to be
required for some reason if *only* TLS v1.3 is supposed to be enabled.

querying via get_options gives us the union of
- system-wide openssl defaults
- our internal SSL defaults
- flags configured by the user via /etc/default/pveproxy

note that by default only 1.2 and 1.3 are enabled in the first place, so
disabling either leaves a single version being set as min and max.

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