Spike to Support Java 15

User story

As a java user, I want to use the latest java version for my Camunda projects.

Scope

Before we can support Java 15, we must investigate what it takes to do that, as Java 15 removes the Nashorn scripting engine.

Acceptance Criteria

• We are know all problems with Java 15 in the product (e.g. that Javascript tasks won't work out of the box anymore)
• We have a recommendation to which extent we can support our users in migrating to Java 15 if they use functionality (spectrum: documentation only to out-of-the-box integration with e.g. GraalVM and perfect backwards compatbility)
• We know most problems in our CI with Java 15 (e.g. that we need to deal with test cases that test Javascript tasks)
  The scope is this ticket is to evaluate what it takes to support Java 15 in Camunda Platform and provide our users a smooth transition.

Background

With Java 15, support for Nashorn - the Javascript scripting engine - is dropped. Before adding support for Java 15, we need to identify a seamless way for our users to keep using their javascript scripts and therefore want to investigate a replacement javascript scripting engine

• Java 15 is supported
• Ideally, customers can keep using their Javascript scripts seamlessly, but we need to make a reasonable decision of effort vs gain