Javadoc for 3.1.3 and higher doesn't work (build with JDK 21)

[robtimus]

Basically a regression after #1561 was fixed. Version 3.1.2's Javadoc jar file is built correctly, but then for 3.1.3, 3.1.4 and 3.2.0 the jar file is missing files again.

[cowtowncoder]

Uhhh. The problem is this is not CI problem but release problem -- I have not used JDK 21 for mvn release workflow, and not need to remember to do that.

[cowtowncoder]

Ok: fixing via #1626 -- use Maven Enforcer plug-in to make sure JDK 21 (or above) used for Maven release plug-in (prepare, perform).

[robtimus]

Excellent, thanks!

[cowtowncoder]

@robtimus Thank you for the follow-up, I dropped the ball here. But hopefully no more :)

[robtimus]

No problem, it wouldn't be a problem if JDK 17 weren't broken.