[The `macOS-10.15` image in Github Actions is deprecated since May, and scheduled for deletion by 30 Aug 2022.](https://github.blog/changelog/2022-07-20-github-actions-the-macos-10-15-actions-runner-image-is-being-deprecated-and-will-be-removed-by-8-30-22/) There's also a warning to that effect in our [latest release's CI run.](https://github.com/diffblue/cbmc/actions/runs/2713358253) We need to move the jobs to `macOS-11` or later by that time, or we're going to have failing jobs.