CentOS 7 IPv6 doc changes

It advertises C7 as an IPv6-compatible platform.
This is possible thanks to fixes in [1] and [2].

[1] https://review.opendev.org/699458
aka 7054b27d
[2] https://review.opendev.org/699172
aka 908bffcf

Change-Id: Ia353a1663a16f48ac83e5ee9a2cf1d6e183ac3a3
Closes-bug: #1848444
Closes-bug: #1848452
Related-bug: #1856532
Related-bug: #1856725