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Is your feature request related to a problem? Please describe.

This issue comes from #7400 (comment)

My ISP seems to give me a IPv6 address via DHCPv6. But they also send one via SLAAC (I think this may be on error).
Prior to 24.7 my OPNsense router couldn't access the internet via IPv6 unless I ran ifconfig igc0 inet6 <SLAAC IPV6 ADDRESS> -alias

When 24.7 was released, this problem went away. When 24.7.1 was released this behaviour came back.
It seems related to 02ae897

It would be nice if this change was a conjugation option I could set in the UI

Describe the solution you like

UI change allowing me to flush SLAAC WAN IPv6 address

Describe alternatives you considered

1. Manually reverting 02ae897
2. Running ifconfig igc0 inet6 <SLAAC IPV6 ADDRESS> -alias

Additional context

Related to discussion here: #7400 (comment)