net-p2p/prowlarr: tell .NET to disable ipv6 when system has not configured it

As stated in
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=259194#c17
.NET 6+ uses dual mode sockets to avoid the separate AF handling.

This breaks on systems (jails) where ipv6 is not enabled. This patch
detects this on prowlarr starts and disables .NET ipv6 support if no
ipv6 is configured.

PR: 260645
Reported by: Michiel van Baak <michiel@vanbaak.eu> (maintainer)