add ca-certificates as required dependency

add ca-certificates as required dependency

ca-certificates provides the necessary root ca certificates
to connect to the PVE enterprise repositories
The package was always installed since pve-manager 3.1-13 as an indirect
'Depends:'  via liblwp-protocol-https, but could have been missing in previous
versions of pve.

Adding this dependency directly makes it sures that it's available
in each PVE installs no matter what Debian packagers do.
With this package installed we no longer need to ship
our own server CA cert.