Add macos-next image

What does this MR do?

Implement the next image from https://gitlab.com/gitlab-org/ci-cd/shared-runners/macos/-/tree/master#macos-and-xcode-version-support. It is built from scratch with the latest tool versions. When can use it to keep the provisioning fresh and as a starting point for a new release once Monterey is stabilised.

Along the way, I had to figure out a number of problems with job dependencies and rules, to get these jobs to run.

Commits are individually reviewable.

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?