Java 15 support

[apupier]

Latest release, as of 28 January 2021, is Java 15. Now that fixes have ben provided for Java 14 #913 . It would be nice to upgrade to Java 15. Given that Java 14 is not an LTS, I think we can remove the Java 14 job and replace by Java 15. (next LTS will be Java 17 planned for September 2021).

//cc @orpiske

• Replace JDK 14 by JDK 15 in matrix job

[orpiske]

I like the idea @apupier, +1 from me.