Refactor macOS CI so that tests run in parallel
- Also:
- Fix bootstrap issue where curl was failing for a month (so obviously not needed)
- Change Windows test stages to use PowerShell Core (where we can) to avoid compat issues
## PR Context
This allows running all tests in about the same about of clock time.
This removes some of the last mentions of the AppVeyor name. Don't change the title, we don't want the word to appear in the CLs
Also problem: the last references are in the changelogs but I don't want to modify them without committee approval.
## PR Context
it was removed, #8686
Split Windows CI into the following Jobs
1. Combine Build and xUnit, which published build artifact
1. Elevated CI tests
1. Unelevated CI test
1. Elevated everything else tests
1. Unelevated everything else tests
1. Package build and test
1. Verify xUnit result
## PR Context
Before this test:
- A CI run would take ~14 minutes
- A Feature run would take ~30 minutes
After this change, it always runs all tests and takes ~20 minutes.
Job `Unelevated everything else tests` is the long poll, taking ~4 minutes longer than the next shortest job. We should be able to get this back down to ~15 if we split that test job somehow.
