Add JDK 15 EA to testing rotation

[jaymode]

The OpenJDK project has an EA (early access) build for JDK 15 available at https://jdk.java.net/15/, which was released on Dec 27, 2019. This issue is to track getting this JDK integrated into CI for testing.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Build)

[mark-vieira]

I've opened https://github.com/elastic/infra/pull/17056 to add the JDK 15 builds to the JVM catalog.