Report Proxmox version

Proxmox is basically layered on top of Debian Linux, and is reported as Debian (since #52 (closed) / !30 (merged)). I think it would be nice to report the Proxmox version, instead of the Debian GNU/Linux 11 (bullseye) it reports now.

Running pveversion provides a string that could be passed through directly (pve-manager/7.2-11/b76d3178 (running kernel: 5.15.60-1-pve)) or trivially processed into something short and sweet (e.g. Proxmox VE 7.2-11 (5.15.60-1-pve)).

Currently the OS is detected at service start. Minor Proxmox upgrades don't need a reboot, so updating 7.2-10 to 7.2-11 wouldn't be picked up until mqtt4hass (or more likely the whole system) is restarted. I am not sure this is a problem that needs solved though.

pveversion is just a Perl script (source), so we could theoretically cut out the middleman and hit the APIs, but that is probably more complex than looking for and executing pveversion.

@qwindelzorf any thoughts/concerns?