CI Failure with JDK 15

[ArquintL]

Using the following JDK in a GitHub action results in non-deterministic failures of AsyncCoreServerSpec test cases. They seem to be caused by Silicon being interrupted and cannot be reproduced locally.

The following step is used to install the mentioned JDK version:

- name: Set up JDK 15
  uses: actions/setup-java@v1
  with:
    java-version: '15.0.1'
    architecture: x64

[ArquintL]

CI has changed a lot since creating this issue. In particular, Java 11 (LTS) is now used

[aterga]

@ArquintL Is there an easy to explain reason why we don't support Java 15? Because that's the version of Java I'm using locally.