advance dhcprelay to 6a6ce24, add default dhcpv6 dualtor source interface (sonic-net#15864)

sonic-build image side change to fix source interface selection in dual tor scenario.
dhcprelay related PR:
[master]fix dhcpv6 relay dual tor source interface selection issue sonic-dhcp-relay#42

Announce dhcprelay submodule to 6a6ce24([to invoke sonic-net#40 PR]([master]fix dhcpv6 relay dual tor source interface selection issue sonic-dhcp-relay#42))

Author: jcaiMR

dockers/docker-dhcp-relay/dhcpv6-relay.agents.j2

@@ -13,7 +13,7 @@
 [program:dhcp6relay]
 command=/usr/sbin/dhcp6relay
 {#- Dual ToR Option #}
-{% if 'subtype' in DEVICE_METADATA['localhost'] and DEVICE_METADATA['localhost']['subtype'] == 'DualToR' %} -d{% endif %}
+{% if 'subtype' in DEVICE_METADATA['localhost'] and DEVICE_METADATA['localhost']['subtype'] == 'DualToR' %} -u Loopback0 {% endif %}
 
 priority=3
 autostart=false