Apache Spark has embraced the GA support for 2.12 since the version 2.4.1, that was released nearly a year ago.
The Amazon EMR v6.0.0 characterised by the adoption of Hadoop 3.2.1 (sic!) and Scala 2.12 (although the default JDK is still Coretto 8) has been released a month ago, hence I presume it would be nice to have cross-builds for the Lighthouse.

This PR was cherry-picked from datamindedbe#27, where you can observe the matrix builds, as currently the Circle CI config supports only 2.11.

Changelog:
- enable cross-version builds for the projects;
- fix compile-time warning.