Issue building with JDK 16

[Thihup]

When using JDK 16 to build this project it fails with the following error:

[ERROR] /jfrunit/src/test/java/dev/morling/jfrunit/StackTraceDisabledSampleEvent.java:[26,8] cannot access jdk.internal.event.Event
  class file for jdk.internal.event.Event not found
[INFO] 1 error

I'll try to get more information about it

[gunnarmorling]

Yes, I believe this is due to using --release=15. I'm not quite sure how it can be done in a more version agnostic way.

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[aalmiray]

Perhaps configuring Maven toolchains along with a profile?

[gunnarmorling]

I've created #68 for moving to JDK 16 as a baseline. Supporting 14 or 15 doesn't make any sense at this point, given they are EOL by now. Adding support for the previous LTS (11) is tracked via #5.