3.0.0-M4 - Still before the configuration redesign

Milestone release with fixes and improvements still before the larger configuration redesign.

❗❗❗ Important Note: ❗❗❗

This bugfix is required for all users of Californium 3.0.0-M1 - 3.0.0-M3,
which are using DTLS RPK/x509 for authentication! It provides the fix for

CVE-2021-34433