Part of the classes is compiled with Java 8 and the rest with Java 11

[pashashiz]

If we take the latest flink-adt_2.12-0.6.1.jar and look inside:

file io/findify/flinkadt/api/serializer/ScalaCaseClassSerializerSnapshot.class
io/findify/flinkadt/api/serializer/ScalaCaseClassSerializerSnapshot.class: compiled Java class data, version 55.0 (Java SE 11)

file io/findify/flinkadt/api/typeinfo/CaseClassComparator.class
io/findify/flinkadt/api/typeinfo/CaseClassComparator.class: compiled Java class data, version 52.0 (Java 1.8)

Any chance we could compile it all with Java 8?