Add openjdk 15 to Java matrix testing rotation

[mark-vieira]

Adding the EA release of Java 15 to our Java matrix testing configuration.

Closes #50764

[elasticmachine]

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

[mark-vieira]

@jasontedor looking at what we've done for other Java versions I assume this needs to be backported to 7.x as well?

[jasontedor]

Yes, and we want to test 6.8 against it as well.

[jasontedor]

@mark-vieira Note that we would want to also add JDK 15 to the build matrix, for 7.x and master, so that we ensure that we can compile/build Javadocs with the JDK 15 compiler (since that will be the future compiler that we use, when JDK 15 is released).

[mark-vieira]

@jasontedor We aren't even doing this for Java 14 yet and Gradle still doesn't officially support JDK 14 either so far as I can tell. I'm afraid this would just add a lot of noise. I'm not sure testing with a JDK that we know not all of our tooling full supports is particularly valuable.

[jasontedor]

Thanks, I forgot that JDK 14 wasn’t functional on Gradle yet.