Enable IPv6 by default

[DerEnderKeks]

Describe what you are trying to accomplish and why in non technical terms
I want that IPv6 is enabled by default, as IPv4 is a legacy standard, IPv6 should always be enabled/supported unless intentionally disabled.

Describe the solution you'd like
IPv6 should be enabled by default again, like it was in the v14 betas. The current behavoir should thus be inverted: Those that have weird setups with hosts that have IPv6 completely disabled should apply a patch to remove the v6 listening directives.

Describe alternatives you've considered
There are no alternatives to this. IPv6 is the most recent standard and thus should be enabled by default. Having it disabled by default just hurts the IPv6 adoption and causes additional effort for anyone with a proper setup.

Additional context
According to #11684, some had issues with this because of their legacy/snowflake setups with completely disabled IPv6.
As this is a very non-standard configuration, this can't be a reason to make everyone else enable the normal behavior again. Any properly configured host isn't impacted by a listener that listens on v6, as all current OSs also listen on v4 by default. Disabling IPv6 entirely on a host is discouraged and people that do so should rightfully suffer [the consequences].

[BobTB]

Most people are not capable of managing / using IPv6, it should be optional as it is now. Never enabled by default. IPv6 is a horrible thing anyway.

[DerEnderKeks]

@BobTB you don't seem to understand IPv6, as it's literally better in basically every way over legacy IPv4, so I'm not going to take your remark very seriously, but I'll try to explain it anyway:

The default behavior of IPv6 [sockets] makes it optional anyway. If your network doesn't have an IPv6 enabled router, you won't notice it in any way and can keep using your legacy v4-only network like it's 1990 just fine. Applications having support for IPv6 is only a problem when you intentionally cripple your system to have IPv6 completely disabled, as that breaks the dual stack sockets, which primarily listen on v6 but also automatically on v4.

Also, what capabilities do you require to manage IPv6? Any half decent consumer router will enable it by default, effectively requiring no setup at all.