Don't configure IPv6 addr/gw when IPv6 disabled.

[robmry]

- What I did

In 25.x, a container started with --sysctl=net.ipv6.conf.all.disable_ipv6=1 did not end up with IPv6 addresses on its interfaces.

#47062 delays setup of network interfaces until after the OS sandbox has been created, so that the container can be inspected for a ::1 interface (initially, in order to exclude IPv6 entries from /etc/hosts). So, interfaces were previously configured after the runtime has applied the sysctl setting, now they're configured afterwards.

Until #47406 makes it in, IPv6 addresses are still allocated if a container is connected to an IPv6 network. The code that adds an IPv6 address also enables IPv6 on the interface.

That didn't matter when it happened before the runtime applied the sysctl to disable IPv6, now it does.

- How I did it

Don't add an IPv6 address or gateway to a container that's got IPv6 disabled.

- How to verify it

New integration test, without this change it fails with ...

=== RUN   TestDisableIPv6Addrs/IPv6_disabled
    bridge_test.go:659: assertion failed:
        --- inet6
        +++ →
          []string{
        - 	"inet6 fda0:ef3d:6430:abcd::2",
        - 	"inet6 fe80::42:acff:fe18:2",
          }

- Description for the changelog

Do not attempt to configure an IPv6 address or gateway in a container that's got IPv6 disabled.