Fix IPv6 route add/delete message log level

Fix IPv6 route add/delete message log level

We have D_ROUTE for route addition/deletion messages, which prints at
loglevel 3. Use that for IPv6, like we do for IPv4 to reduce terminal
spam for non-legacy-networking setups. Prvious code would print the
messages at --verb 1.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20240105135742.21174-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27954.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit b959b02b4f607628896b4092f7ddfa675e87d929)