You can set LXD_INSECURE_TLS=true in the environment of the LXD 5.0 system.
Easiest way to do this would be systemctl edit snap.lxd.daemon and then add a section like:
[Service]
Environment=LXD_INSECURE_TLS=true
To get it applied, you could try systemctl reload snap.lxd.daemon which may do the trick. If it doesn’t, a full system reboot is likely the easiest alternative.