Update californium CoAP server

Qt CoAP uses docker containers based on a californium test-server for running tests, and also as a counterpart for the Secure CoAP Client example.

The problem is that the PSK-based encrypted communication does not work anymore.
The request ends up with the following error on the Qt side:

qt.coap.connection: Handshake error:  "Error during SSL handshake: error:0A000152:SSL routines::unsafe legacy renegotiation disabled"

This might be because the old versions of californium server do not support renegotiation.

Let's try to update the californium server to version 3.8 and check, if it fixes the issue.

Currently, the server which is used for the docker containers is located here: https://github.com/sonakur/californium

It has only one custom commit, which we need to cherry-pick to the newer server as well.

The internal docker image uses this Dockerfile, and is based on this provisioning script.

The docker container used for the Secure CoAP Client example is located here.