Deprecate Java 8 and move to Java 11

Right now we support all Java version starting on Java 8.

Soon, we will have to move towards Java 11 as some of our dependencies are no longer supporting Java 8 (e.g., [mockito](https://github.com/SpoonLabs/flacoco/pull/221), [spoon](https://github.com/SpoonLabs/flacoco/pull/132)).

The issue with this is that some benchmarks, including https://github.com/rjust/defects4j, require Java 8.

Start by reading the linked Mockito and Spoon dependency changes, then review how the project supports Java versions and how the Defects4J benchmarks are invoked. Done means the Java 11 migration path and the Java 8 benchmark compatibility decision are documented and implemented, but this issue names no files or tests.

Written by the indexing model from the issue text.