add a JDK 15 build #1225
Description
It's time!
(Aside: while I'm doing this, I think I would like to rename the JDK 8 Jenkins jobs to have -jdk8- in the name, since on 2.13.x the -jdk11- build is now the "main" build — or perhaps there isn't any "main" build anymore, since we now have at least one repo that fails in all three builds (fs2 fails in JDK 8, shapeless-java-records fails on 8 and 11, and several repos still fail on JDK 14 just because the maintainers haven't dealt with it). There is still a "main" build in the sense of "JDK version I typically test on", but it's no longer consistent across Scala versions which one that is: 2.11.x and 2.12.x remains 8, but on 2.13.x I do 11 first, and check 8 later.)