chore: pin macOS runner to version 14 for stable iOS builds

[amoinier]

This PR pins the macOS runner version to 14 to ensure reliable iOS builds in CI/CD pipelines.

Why this change:

• macOS 15 runners have incomplete iOS SDK installations causing build failures
  ([macos-15] iOS build fails on CI due to incorrect SDK targeting actions/runner-images#12758)
• macOS 14 provides stable, tested iOS development environment

Why version pinning is good practice:

• Reproducible builds: Ensures consistent build environment across all CI runs
• Prevents breaking changes: Avoids automatic upgrades that can introduce compatibility issues
• Faster debugging: Eliminates environment variables when troubleshooting build failures
• Team alignment: All developers and CI use the same tested configuration

[mateusz-bak]

Makes sense, thanks!