Eclipse Californium (Cf) CoAP Framework 2.6.4

Bugfix release:

• 9900dfde6 2021-07-12 Add backOffRetransmission to DtlsConnectorConfig.clone.
• 688b22dca 2021-07-11 Fix URI encoding of scoped IPv6 hostnames.
• 9d92f806a 2021-07-10 Fix NullPointerException on missing network interfaces.
• f65031de4 2021-07-05 Update the signer-plugin.
• 9fbc2c2bf 2021-06-08 Fix anonymous RPK handshakes.
• 1888b40f8 2021-06-03 Improve checking for invalid URIs.
• 2373fae7d 2021-05-25 Use unexpected message for not expected handshake message type instead of internal error.
• 675245c80 2021-05-09 Fix dead-lock in dtls shutdown.
• 1ed12ba06 2021-05-06 Fix typos in javadoc.
• a62c71650 2021-05-06 Fix copy and paste error in logging message and javadoc.