Connecting to legacy LXD server via remote

[despens (Dragan Espenschied)]

Thank you Stéphane, this was the final missing hint. These are all the steps I needed to do:

1. Try to connect to the remote LXC host from the incus host one time. It will fail, but will generate a client certificate:
   incus remote add <remote_name> <remote_addr>

2. Copy the file ~/.config/incus/client.crt to the LXC host. It is always in the home directory of the user you’re establishing the remote connection with.

3. On the LXC host, import the client certificate:
   lxc config trust add client.crt

4. On the LXC host, unset any token identification that might have been set up:
   lxc config unset core.trust_password

5. Repeat the connection attempt on the incus host:
   incus remote add <remote_name> <remote_addr>

Et voilá!