Ignore JUnit 6.x upgrade proposals

[ppkarwasz]

Changes the Dependabot configuration, to ignore JUnit 6 upgrade proposals. JUnit 6 requires Java 17 and most of the projects have a Java 8 baseline.

[garydgregory]

While it's true that the majority of components are stuck on Java 8 (but not all) for the foreseeable future, I would prefer to (1) keep being updated at to what's new and (2) keep the CI simpler.

[ppkarwasz]

I think that adding exclusions to dependabot.yml is a self-documenting way to know what to change, when we finally migrate to Java 17. What do you think?

[garydgregory]

Not really, it hides the fact that updating to 6.x requires Java 17.
For me, seeing it red reminds me that we are stuck on Java 8 for the foreseeable future (until Java 8 goes down in usage some more). Also, simpler here is better IMO.