Change: [CI] Always use latest stable Xcode

[glx22]

Fixes #12340.

Motivation / Problem

Since 14.0-RC2 we use macos-14 which defaults to Xcode 15.0.1, and due to known issues in Xcode 15 the resulting build crashes on macOS12 or older (#12340 using macOS 10.15.7 is affected).
The issue is supposed to be fixed in Xcode 15.1.

Description

Force using latest stable Xcode version (15.2 in macos-14 image).

Draft PR for now, until confirmation it's working, I'll trigger a PR release for testing (forgot it's not possible to test modifications in release workflow).

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[glx22]

Release build using workflow from this PR:
https://cdn.openttd.org/openttd-branches/pr12390/20240327-pr12390-gd750db9bbe/openttd-20240327-pr12390-gd750db9bbe-macos-universal.zip