NS8 join cluster with core version mismatch

[mrmarkuz (Markus Neuberger)]

To update the core of the new node to version 3.21.0:

api-cli run update-core --data '{"core_url":"ghcr.io/nethserver/core:3.21.0","nodes":[1]}'

After the update it should be possible to join the new node to the leader.