Setup native IPv6 connectivity on web-001

Summary:
OVH provides native IPv6 at gra2, so we don't need a HE tunnel.

Routing is slightly more complex as the gateway does NOT belong to our range,
we use a similar configuration than IPFO with two routes, the first to reach
the gateway, the second default one through that gateway.

To determine if we're in such case can be done with code introduced in D3077.

On FreeBSD, to find the gateway through NDP (neighbor discovery protocol)
produces until 13.1 the following error (here with ping):

ping6: sendmsg: No buffer space available
ping6: wrote www.google.com 16 chars, ret=-1

As such, this routing "as is" without any NDP fix requires FreeBSD 13.2+
to benefit from https://reviews.freebsd.org/D23695 change.

References:

• https://en.wikipedia.org/wiki/Neighbor_Discovery_Protocol
• https://datatracker.ietf.org/doc/html/rfc5942 (Introduction and RFC 5942 4.1)
• https://reviews.freebsd.org/D23695

Ref T1853

Test Plan:

• deploy on a machine using HE tunnel, should be no-op
• deploy on a machine not using IPv6, should be no-op
• deploy on web-001, should change /etc/rc.conf.d/routing/ipv6

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1853

Differential Revision: https://devcentral.nasqueron.org/D3078