[Stack Monitoring] Support automatic update of stack version dependencies

[matschaffer]

PRs like #3993 introduce our own stack components as testing dependencies for our packages.

For the moment, I suspect we'll have to manually keep the version updated, or use a moving target like master-SNAPSHOT.

Ideally we'd have something similar to whatever opens PRs like elastic/beats#32720 that update the version when a snapshot gets fully built.

That way we have a forcing function to keep things up to date, but any failures introduced by new stack versions won't cause the main branch of this repo to start failing (only the "bump" PR should fail).

[klacabane]

Right now we'll validate against 8.5.0 and the plan is to update the version when new snapshots are released. It looks like elastic-package allows the definition of multiple versions in a variants.yml file and runs the test suite for each of the version defined. We should use that mechanism and append the new version to that file instead of replacing the old one.

[smith]

This would be nice to have but we're not prioritizing it now.

[miltonhultgren]

@smith We currently have a broken build for the Beats Integration due to some change happening in the Stack that isn't reflected in the tests continuously. Doing the work described in this issue would help us catch such issues early.