Additional configuration needed so that logging alert plugin can be configured

[c8y3]

Since Graylog 5.1.11 (https://graylog.org/post/announcing-graylog-v5-1-11/), for security reasons (GHSA-p6gg-5hf4-4rgj), the classes than can be loaded in the cluster configuration have been restricted (Graylog2/graylog2-server#18165).
In order for the graylog-plugin-logging-alert and graylog-plugin-alert-wizard to work (for both non-regression tests and in production), we need to add this line in the configuration.

safe_classes = org.graylog.,org.graylog2.,com.airbus_cyber_security.

We probably need to document this in the root README.