Bind systemd socket activation to IPv6
By default foreman.socket binds to IPv4 only. This was done to make matching it in Puma easier, but it's not really what should be done in 2021 where everything should be dual stack by default.
In #31431 the need for this matching in Puma was removed which makes it much more flexible. The default should be updated.
Note that the installer currently always overrides it so it's not really a difference for most, but I want to modify the installer to rely on the default.
Given that this is not used by default, I'm setting this as a target for 2.4.1. It can't be picked to an earlier release since #31431 was merged in 2.4.0. This really is fixing something that should have been part of it.