Always run Gradle on JDK 21

[wmdietl]

Gradle 9.0.0 no longer runs on JDK < 17.
#83 updated to 9.0.0. Oddly passed CI.
We need to update the setup to always run with a newer JDK, like we're doing elsewhere.

[wmdietl]

@thisisalexandercook Can you please take a look at this?