Clean up pom.xml + remove project.build.sourceEncoding which is inherited from grandparent pom + specify drools-planner 6.0.0-SNAPSHOT before migrating to optaplanner
Fix compilation errors due transitive deps not being put in compilation path (and they shouldn't be put in the compilation path, but historically maven/intellij has always done this, now apparently it stopped doing that in the maven-compiler-plugin 3.0?)