Use toolchains to ensure compatibility with Java 1.8

Since using Tycho for building Eclipse feature and update site modules only runs on Java 11+, we can no longer mandate to build releases using Java 1.8 to ensure compatibility with Java 1.8.

To fix this problem, to allow Maven to run with Java 11 while still making sure that the compiled code works properly with Java 1.8, we'll have to use the Maven Toolchains Plugin.