Upgrade to jackrabbit 2.20.2

Our current jackrabbit 2.18.x baseline is going EOL soon: https://issues.apache.org/jira/browse/JCR-4552

So we need to rebase and upgrade our hippo-jackrabbit project onto latest jackrabbit-2.20.2.

I briefly reviewed the differences between 2.18.5 and 2.20.x last April, where I noticed one complication:

The main problem for us is not the upgrade to v2.20 itself, as indeed there is little/no technical difference with v2.18.

But what is impactful is that they moved the jackrabbit-api module 'upstream' into oak (so as of 2.20+ jackrabbit now is a downstream project of jackrabbit-oak 1.20+ ...).
Code-wise the new oak-jackrabbit-api module is still fully compatible with the old jackrabbit-api module, but the dependency coordinates have changed.
Which means upgrade steps, at least for us (adjusting the dependency management in the release-pom, as well all our projects depending on it (cms and repository).
And it means that customer projects no longer get the jackrabbit-api module either, but the new oak-jackrabbit-api instead.
If they somehow (but unlikely) have their own usage of the jackrabbit-api dependency, their project no longer will build until they adjust that dependency too.
Meaning: we'll have to provide upgrade instructions for this, also for minor/patch releases, when we backport the upgrade to jackrabbit 2.20.x

 Part of this rebasing effort is to update the notes and instructions to do so, so it becomes easier going for others to repeat this in the future.