Support JDK 21

Currently, we run the scripted tests with sbt 1.5.2.

The first version of sbt that works with JDK 21 out-of-the box is sbt 1.9.0 (released in June 2023).

We should consider running the scripted tests with sbt 1.9.0: it improves the contribution experience, and given the choice I'd rather catch problems/regressions running the plugin with Java 21 than running the plugin with sbt <1.9.0