Add the "bootfile URL" DHCP option to https://docs.redhat.com/en/documentation/red_hat_satellite/6.16/html/provisioning_hosts/configuring_networking_provisioning#DHCP_Options_provisioning 

IPv6 is now supported not only in UEFI HTTP boot, but also in PXE TFTP boot as well as bootdisk and Discovery.

Unmanaged DHCP for IPv6

If you want to provision in IPv6-only network, the DHCP server has to respond with the "bootfile-url" option.
https://kb.isc.org/docs/standard-dhcp-options#dhcpv6-options-updated-20240129
no. 59 Bootfile URL
The URL has to point a first-stage bootloader on TFTP Capsule, such as pxelinux.0

Equivalent of next-server in IPv4
We can't set it by foreman-installer, the user has to do it manually

We will support PXE over TFTP, Discovery and bootdisk as well