Update the build workflow to use macos-13 and Xcode 15

[bjornoleh]

We need to update the build workflow to use macos-13 and Xcode 15, as iOS 17 SDK will soon be required at App Store Connect.

A quick overview of SDK versions is available here:
https://xcodereleases.com/?scope=release

GitHub macos-13 runner, Xcode versions:
https://github.com/actions/runner-images/blob/main/images/macos/macos-13-Readme.md#xcode

Please see email from :

Relevant lines:

LoopFollow/.github/workflows/build_LoopFollow.yml

Lines 23 to 27 in 5e3a016

	runs-on: macos-12
	steps:
	# Uncomment to manually select latest Xcode if needed
	- name: Select Latest Xcode
	run: "sudo xcode-select --switch /Applications/Xcode_14.1.app/Contents/Developer"

Probably there are other workflow changes to import from Loop as well, as LF is running a little behind now.