Upgrade notes for agent identification through fingerprints

Requires backend#1896 (closed), enterprise#49 and ponos-agent#77

The upgrade notes for the next release should mention that Ponos agents in Enterprise Edition now use a host fingerprint to identify themselves with the backend, which implies that:

• The private_key setting must be removed from the agent configuration
• The agent private key files can be deleted
• The backend's private key must not be deleted, as it is still used for secrets