Upgrade KE to Java version 21

[bnouwt]

Currently, the KE requires Java version > 17 and some latest version of libraries the KE relies on (like Apache Jena 6.0.0) require at least Java version 21.

So, after resolving this issue, we can also merge this PR: #798

[bnouwt]

See this page for some of the improvements between Java version 17 and 21.

[bnouwt]

Make sure to check whether our docker images still work, since they might not use Java 21.