[AWS EC2] cloud-init fails to bring up ipv6 on RHEL 8 and RHEL 9 images

Issue

• IPv6 address is not assigned to the interface after configuring cloud-init /etc/cloud/cloud.cfg.d/99-custom-networking.cfg file with dhcp6

• cloud-init should add the following parameters to the network interface configuration file:

    # cat /etc/sysconfig/network-scripts/ifcfg-eth0 
    # Created by cloud-init on instance boot automatically, do not edit.
    #
    BOOTPROTO=dhcp
    DEVICE=eth0
    DHCPV6C=yes  <<-----
    IPV6INIT=yes <<-----
    ONBOOT=yes
    STARTMODE=auto
    TYPE=Ethernet
    USERCTL=no