Minimal Requirement for Leshan.

Leshan 2.x

Component	Minimal Requirement
Maven Build	Maven 3.6.0 Java 17
Leshan library	Java 8
Leshan client demo	Java 8
Leshan server demo	Java 17 (because of jetty 12 dependency)
Leshan bsserver demo	Java 17 (because of jetty 12 dependency)