Compilation is only possible now in JDK 21 due to apache derby test dependencies

[nmondal]

Because apache derby has a security vulnerability, we migrated it to latest version, that does not have any. But this version is only Java 21 compatible. Thus, we have a problem building the entire system with tests in Java 17.

We can of course build the system w/o running or compiling the tests in Java 17.

[hemil-ruparel-blox]

I think it makes sense to have java 21 as minimum supported JDK for COWJ. It is widely available in all distributions + it is LTS

[nmondal]

This is not actually true. If the tests were to build and run, then ONLY requirement of Java 21 arise. Rest, it can build on Jdk 17 as well.