Eclipse Californium (Cf) CoAP Framework 2.6.6

Bugfix release.

• f2a335546 2021-11-09 Fix null-pointer on unknown signature- and hash-algorithms.
• aaf83280c 2021-11-04 Adapt unit test to use new certificates.
• cf3d4e384 2021-11-04 Update demo-certs.
• 10c9e70c5 2021-10-29 Fix simple file-server for 2.6
• 1cb2e8a34 2021-10-29 Fix non-existing.host now exists.
• ae761e652 2021-10-29 Update plugins and test dependencies.
• 47a3a8dfe 2021-10-03 Fix clearAndNotifyObserveRelations.
• 8228c2f39 2021-10-03 Add unit test for delete with observe relation.
• 110e169ad 2021-10-03 Extend to exclude openssl 1.1.1 server on windows from the interoperability tests.

Includes all fixes in order to build Californium locally, supporting java 16 + 17 as well.