Upgrade to MacOS 12, Xcode 13

Since yesterday it's been very hard to get all MacOS jobs to pass CI. They fail during different brew operations. See e.g. https://gitlab.com/graphviz/graphviz/-/pipelines/437025207.

Based on https://gitlab.com/gitlab-com/macos-buildcloud-runners-beta/-/issues/3#note_774084791, I was hoping that upgrading would help. Perhaps it does, but the jobs fail for other reasons, hence the draft status.

I don't have time to look into this today so if anyone else wants to pick up the baton, this task is up for grabs.