MacOS 13 GitHub Actions runner is being retired soon

[anoadragon453]

https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down/

Brown-outs will begin starting November. We make use of this runner in our release workflow:

synapse/.github/workflows/release-artifacts.yml

Lines 113 to 118 in 0aeb95f

	matrix:
	os:
	- ubuntu-24.04
	- ubuntu-24.04-arm
	- macos-13 # This uses x86-64
	- macos-14 # This uses arm64

We should drop support for MacOS 13, and add support for MacOS 15. We should continue to build for x86_64 and arm64 where we can.

[sandhose]

Reading the GH blog post, I think this means replacing macos-13 with macos-15-intel to continue building x86_64

[sandhose]

And the cibuildwheel docs seems to agree with me: https://cibuildwheel.pypa.io/en/stable/ci-services/#github-actions