nixos/release-notes: mention that dhcpcd stopped giving IPv4 addresses to bridges by default

This is an backward incompatible change from upstream dhcpcd [0], as
this could have easily locked me out of my box.

As dhcpcd doesn't allow to use only a blacklist (denyinterfaces in
dhcpcd.conf) of devices and use all remaining devices, while explicitly
allowing some interfaces like bridges, I think the best option would be
to not change anything about it and just educate the users here about
that edge case and how to solve it.

[0] https://roy.marples.name/archives/dhcpcd-discuss/0002621.html

(cherry picked from commit eeeb2bf8035b309a636d596de6a3b1d52ca427b1)