Use macos-latest image for CI builds

[mjcheetham]

The macos-10.15 image for CI builds is deprecated on GitHub Actions. We
should update this to a supported image.

Given we are actually running our release builds on macos-latest, we
should probably be running CI on the same image too!

Fixes #802

[dscho]

Looks good! I guess we only have to be mindful when new generations of macos-* build agents become available, and pivot quickly if builds require corresponding adjustments.

[dscho]

@mjcheetham FYI I adjusted the branch protections accordingly.

[dscho]

@mjcheetham FYI I adjusted the branch protections accordingly.

I guess we'll want to require the CodeQL checks, too, right?

[mjcheetham]

Looks good! I guess we only have to be mindful when new generations of macos-* build agents become available, and pivot quickly if builds require corresponding adjustments.

Yeah. The CLR abstracts most of the OS, but for the direct native interop bits that we use it'd be good to get early breakages for anything that needs reaction.

[dscho]

Looks good! I guess we only have to be mindful when new generations of macos-* build agents become available, and pivot quickly if builds require corresponding adjustments.

Yeah. The CLR abstracts most of the OS, but for the direct native interop bits that we use it'd be good to get early breakages for anything that needs reaction.

Done.