Shellspec jobs and workflows fail because no MacOS (10.15) runner is available

[bencomp]

What steps does it take to reproduce the issue?

• When does this issue occur?
  This happens on GitHub Actions when the Shellspec workflow is triggered through a push or pull request.

• Which page(s) does it occurs on?
  n/a

• What happens?
  The Ubuntu and CentOS jobs succeed in less than 30s, but the MacOS job is queued for 1.5 day and then automatically fails because no runners are available.
  I am pretty sure the cause is that the job is specified to run on macos-10.15 runners, which are not listed under Choosing a GitHub-hosted runner. MacOS 10.15 is end-of-life.

• To whom does it occur (all users, curators, superusers)?
  Developers who look at workflow runs or who receive emails that a job has failed.

• What did you expect to happen?
  The MacOS job should pass as well.
  (This could be another issue, but it looks like newer commits on a branch do not cancel pending/queued jobs. I would expect that jobs are cancelled when a commit is outdated.)

Which version of Dataverse are you using?
The develop branch, or a feature/fix branch based off of the develop branch.

Any related open or closed issues to this bug report?
I could not find any issues related to the shellspec workflow.

Screenshots:

[pdurbin]

Huh. Interesting! Thanks for the bug report, @bencomp! I didn't realize this is happening. 🤔

[bencomp]

I found out because updating the branch for #9068 on my fork triggered this workflow to run and fail. At first I figured that there were just fewer MacOS runners, but then I saw 10.15 in the workflow file and realised Homebrew on my MacBook Pro running OSX 10.15 (the one that I had when I visited the IQSS) is always complaining that Apple and Homebrew don't support that old version anymore.

My guess is that this can be easily fixed, by changing the runner to a newer MacOS version, or to macos-latest. If support for 10.15 is important, I think you need to set up a self-hosted runner.