Run upgrade tests on PRs and pushes to trunk

[johnbillion]

We've got ​an "Upgrade Tests" GitHub Actions workflow which runs upgrade tests, but this workflow only runs when manually triggered, it doesn't run on PRs or pushes to trunk (unless the workflow file itself is changed).

There should be a workflow which tests the process of upgrading from a prior version to a Zip built from the current branch.

The existing "Upgrade Tests" workflow tests upgrades from a given version to the latest stable. It needs to be adjusted (or we need a new workflow) which allows upgrading to a specified Zip file.

To keep the number of tests (and the workflow time) manageable, we shouldn't test the entire matrix, but just a few choice upgrades.