Baseline Java 17

[gaeljw]

This issue is to track the discussion and work that started in #488.

The ecosystem has moved on and we should do as well: Java 17 should be the minimum version now.

[gaeljw]

I was not aware of that but this implies getting rid of Scala 2.12 as well ( #689 ) because Scala 2.12 cannot generates Java 17 class files.

This is the warning raised by the compiler when doing so and it indeed still generates Java 8 class files.

Scala 2.12 cannot emit valid class files for targets newer than 8; this is possible with Scala 2.13