Switch to Java 21

[tim-we]

We should switch to the current LTS version of Java, which at the time of writing is Java 21.

https://mydeveloperplanet.com/2023/11/01/whats-new-between-java-17-and-java-21/

[tim-we]

Seems like we have to wait for new Mockito release that is compatible with Java 21. Currently the test are failing because Mockito fails to mock classes.

[tim-we]

Try to change pom.xml from

<maven.compiler.source>17</maven.compiler.source>
<maven.compiler.target>17</maven.compiler.target>

to

<maven.compiler.release>17</maven.compiler.release>

Make sure this works in the CI pipeline, there were some issues with the maven compiler plugin.