Add migration message in maven-clover2-plugin POM

investigate if we can use clover-maven-plugin or must use cloverX-maven-plugin > it would be great to have just clover instead of cloverX- to avoid changing a number in future
decide if major Clover release will be numbered 4.1.0 or 5.0.0 (as per Semantic Versioning) > it may affect whether it will be named clover4 or clover-5
prepare migration message in maven-clover2-plugin pom.xml for versions 4.0.x, similarly as it was done for Clover Core (com.cenqua.clover:clover:3.3.0) -> deliver in Clover 4.0.6, 4.0.7, ...

to be considered:
prepare a fork of the plugin and release under two names since 4.0.6?
add an extra migration warning printed by the clover2:setup / clover2:instr goal?