pve-firewall.service: update-alternative ip-/eb- tables to legacy versions

pve-firewall.service: update-alternative ip-/eb- tables to legacy versions

This is rather a bit of an hack but works for us for now.

we need to use the legacy versions for both due some bugs in the
nftables based versions, i.e., for iptables it's Debian bug #929527 [0]
and for ebtables it's Debian bug #929976 [1]. While the first gained
some response from the maintainer and a solution is in sight it's
currently blocked by Buster release freeze policy. The second one did
not get any response so far.

[0]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=929527
[1]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=929976

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>