Upgrade to Java 11

Java 11 is currently (April 2021) the latest Long Term Support (LTS) release of Java, until Java 17 is released later this year (September 2021).

This must be done together with upgrading to a newer Eclipse version (see #28 (closed)), as Eclipse 2020-06 doesn't work well with Java 9 (or higher). Java 9 or higher became mandatory as of Eclipse 2020-09.