Validate nftables rule files in CI and fix IPv6

[mwindower]

Description

This checks our nftables rule test fixtures against nft -c -f in the CI.
Thereby some minor issues withe the IPv6 use case were seen and fixed.

[majst01]

Validation failed, but ci passed ?

/testdata/nftrules_ipv6:15:46-54: Error: conflicting protocols specified: ip vs. ip6
        ip saddr 10.0.0.0/8 tcp dport domain ip6 daddr 2a02:c00:20::1 accept comment "dnat to dns proxy"
                                             ^^^^^^^^^
/testdata/nftrules_ipv6:16:46-54: Error: conflicting protocols specified: ip vs. ip6
        ip saddr 10.0.0.0/8 udp dport domain ip6 daddr 2a02:c00:20::1 accept comment "dnat to dns proxy"
                                             ^^^^^^^^^
/testdata/nftrules_ipv6:68:19-36: Error: datatype mismatch, expected IPv6 address, expression has type IPv4 address
        ip6 daddr @proxy_dns_servers iifname "vlan3981" tcp dport domain dnat ip6 to 2a02:c00:20::1 comment "dnat to dns proxy"
        ~~~~~~~~~ ^^^^^^^^^^^^^^^^^^
/testdata/nftrules_ipv6:69:19-36: Error: datatype mismatch, expected IPv6 address, expression has type IPv4 address
        ip6 daddr @proxy_dns_servers iifname "vlan3981" udp dport domain dnat ip6 to 2a02:c00:20::1 comment "dnat to dns proxy"
        ~~~~~~~~~ ^^^^^^^^^^^^^^^^^^
/testdata/nftrules_ipv6:70:19-36: Error: datatype mismatch, expected IPv6 address, expression has type IPv4 address
        ip6 daddr @proxy_dns_servers iifname "vlan3982" tcp dport domain dnat ip6 to 2a02:c00:20::1 comment "dnat to dns proxy"
        ~~~~~~~~~ ^^^^^^^^^^^^^^^^^^
/testdata/nftrules_ipv6:71:19-36: Error: datatype mismatch, expected IPv6 address, expression has type IPv4 address
        ip6 daddr @proxy_dns_servers iifname "vlan3982" udp dport domain dnat ip6 to 2a02:c00:20::1 comment "dnat to dns proxy"
        ~~~~~~~~~ ^^^^^^^^^^^^^^^^^^
Testing nft rules on ubuntu:24.04 with input /testdata/nftrules_ipv6: ❌
nft input /testdata/nftrules_ipv6 on ubuntu:24.04 produces an invalid configura

[majst01]

These errors are my fault

[mwindower]

I'll come up with a fix :-)