[stable] GHA: Switch to old Xcode as workaround for macOS-13 jobs

[kinke]

No description provided.

[dlang-bot]

Thanks for your pull request and interest in making D better, @kinke! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + dmd#16194"

[ibuclaw]

At some point Xcode 14 will be unavailable, so this is just prolonging the inevitable - someone who is intimate with OSX and dmd back-end needs to:

1. Fix this issue in the compiler.
2. Fix all other issues targeting OSX whilst we're at it.
3. Fix debugging on OSX (with lldb/gdb).

[kinke]

There's an extra disadvantage - this workaround would need to be duplicated in the Phobos YAML.

[ibuclaw]

At the very least target stable.

[ibuclaw]

Note: if this gets fixed in dmd, then the host compiler will have to be swapped with ldc until enough time has passed that dmd can bootstrap itself again.

See: #13890