Github are removing macos-11 as a runner - affects iOS build

Github have deprecated macos-11 as a runner, and have said they will remove it on 28 Jun 2024.

See [this Github blog post](https://github.blog/changelog/2024-05-20-actions-upcoming-changes-to-github-hosted-macos-runners/) for the announcement.

Currently, the autobuild workflow for iOS uses the macos-11 runner, so it needs to be updated to use a later macos as the runner instead.