Proposal: Move the build & runtime from Java 17 to Java 21

[aglinxinyuan]

The problem

jOOQ's Open Source Edition tracks the latest LTS JDK — 3.20+ is compiled to Java 21 bytecode (class file 65), but we build and run on Java 17 (class 61). So any jOOQ ≥ 3.20 compiles fine but fails the moment a JVM loads the class.

#6225 hit exactly this: the Dependabot bump to 3.21.6 went red at test runtime with UnsupportedClassVersionError ... class file version 65.0 ... only recognizes up to 61.0 on amber-integration and computing-unit-managing-service. I capped it at 3.19.36 (the last Java-17 jOOQ release) to unblock — but every jOOQ bump past 3.19 is now blocked until we move off Java 17, and other libraries are raising their baseline to 21 too.

Proposal

Move the build & runtime from Java 17 → Java 21 (current LTS). Touches the CI matrices (build.yml, benchmarks.yml, build-and-push-images.yml), the build.sbt javac/scalac target, the Docker base images, and the setup docs.

Java 17 isn't EOL, so no fire — but the library floor is rising and #6225 is the first place it's costing us. This is bigger than a dep bump (needs a green run across engine + services + Docker images), so raising it before scoping a PR. Objections?