The antlr4-maven-plugin using inconsistent dependencies / requirements

[khmarbaise]

The antlr4-maven-plugin is requiring Maven 3.0 via prerequisites and has a dependency to maven-core (test scoped) which is using version 3.3.9 furthermore there is a dependency to the maven-plugin-api with version 3.0.5 and has a dependency to maven-project version 2.2.1.

Steps to clean this up:

1. The first step is to remove the maven-project dependency and change the scope of maven-core from test to compile to have acccess to MavenProject within the Mojo. (antlr4-maven-plugin is using maven-project version 2.2.1 dependency #3096)
2. Upgrade the maven-plugin-api to the same version as maven-core
3. Lifting the minimum required Maven version to 3.3.9 as well. Or maybe to a different level but consistent (prerequisites vs. dependencies). (Prerequisites usages in pom.xml #3102)