Current JDT implementation of MR projects setup differs from "naive"
user expectations in a way project JRE should be setup.

Instead of using lowest target JRE with default JLS settings matching
JRE version, one is supposed to setup highest targeted JRE but with
lowest targeting JLS for release as project default.

Beside changing JRE, removed extra target output folders to simplify
project config.

See https://github.com/eclipse-jdt/eclipse.jdt.core/issues/4849