[midPoint] Maven 3.6 is required to build midPoint

[Radovan Semancik]

Hello,

Apache Maven 3.6 is a minimum requirement to build master branch of 
midPoint.

I have made a lot of upgrades of midPoint dependencies to make sure that 
the Gutenberg can be really a *long* time support release chain. 
Dependency management is never easy as it often leads to conflicting 
dependencies. I have managed to resolve the conflicts and even modernize 
some really old pieces of midPoint. However, the consequence is that now 
we depend on Apache Maven API 3.6. Therefore Maven 3.6 is needed to 
build midPoint. I hope that this will not cause any major trouble as 
Maven 3.6 was released almost a year ago.

Also, upgrade to JDK 11 is strongly recommended for midPoint 4.0 and any 
later versions.

-- 
Radovan Semancik
Software Architect
evolveum.com