net: enable IPv6 by default

As of 2024-04-01, IPv6 represents ~43% of traffic entering Google with
up to 75% is some country, and in generally more available when IPv6 was
introduced in Monero in 2019 as part of 155475d97196 ("Add IPv6
support").

Monero overall has a very low presence over IPv6 which in part can be
explained from the fact that IPv6 need to be specifically enabled before
it is used and often requires nodes to be manually added in
configuration.

This commit enabled IPv6 by default for both RPC and P2P as an attempt
to improve Monero network mesh diversity.

The change has been tested in a lot of different scenario: IPv4 only,
IPv6 only, IPv4+IPv6, IPv4+IPv6 with broken/invalid IPv4 system
configuration, IPv4+IPv6 with broken/invalid IPv6 system configuration.

* --p2p-use-ipv6 is now deprecated and has no effect
* --p2p-ignore-ipv6 is introduced to ignore unsuccessful IPv6 P2P binding
* --rpc-use-ipv6 is now deprecated and has no effect
* --rpc-ignore-ipv6 is introduced to ignore unsuccessful IPv6 RPC binding

See: https://github.com/monero-project/monero/issues/8818
See: https://www.google.com/intl/en/ipv6/statistics.html
See: https://www.vyncke.org/ipv6status/

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