Switch to Java 21

[JohannesLichtenberger]

[AlvinKuruvilla]

Can we make a checklist of everything that needs to be done to switch to Java 18? I just installed Java 18, and besides unused imports and unused variables warnings, everything seems to be okay

[JohannesLichtenberger]

We have to adapt the pom.xml, but other than that Java is luckily in most cases really backwards compatible and it just compiles 👍 Though @mosheduminer did native builds with the GraalVM and as of now it only supports up to Java 17. However, I've read that they are always a few months behind the usual JDK versions.

[JohannesLichtenberger]

Furthermore https://github.com/sirixdb/brackit/blob/master/.github/workflows/build.yml has to be adapted.

[mosheduminer]

I have no problem if you upgrade to JDK 18 while we wait for graalvm to support it. Of course I won't be able to compile brackit to native with the latest fixes until then, but if you want to start using jdk 18 I don't think it's a reason to hold back.

[JohannesLichtenberger]

Now we should switch to Java 20 instead... that is, it's released on 21. march