Update Java, plugins, and dependencies

[nipafx]

Closes #729, closes #726.

Proposed commit message:

Update Gradle, Java, plugins, and dependencies (#729, #726 / #727)

* Gradle: 7.6 ~> 8.1
* Java:
	* regular build: 19 ~> 20 
	* experimental build: 20 ~> 21
	* include 20 in release build to make Javadoc 20 available
	  (and remove the need for multiple Java versions from all
	  other builds to make them simpler and faster)
* all plugins
	* updating Jacoco to 0.8.9 removed an error on JDK 20
* all dependencies
	* stick to same minor JUnit version because that's our policy
	* stick to same major Mockito version because the build failed
	  with 5.2.0

Closes: #729, #726
PR: #727

---

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[nipafx]

I think I'm all done. When everything's green, we can merge.

We can only really find out whether it works when we try a release, though. I'll try to make a meaningful change elsewhere, so a release makes sense.