Update BUILDING.md to disambiguate which stable version of Xcode to build against.

[MrRage]

• [X ] I have searched open and closed issues for duplicates
• [ X] I am submitting a bug report for existing functionality that does not work as intended
• [ X] This isn't a feature request or a discussion topic

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Bug description

During the OS release cycle, Apple may maintain two stable versions of Xcode. Specifying that the latest version on the latest OS helps users identify which Xcode to use without having to maintain versions in the doc. Although I would be more partial to saying which version of Xcode is the supported version.

Steps to reproduce

• Building with the stable version of Xcode 13 produces errors due to Xcode 13 not supporting the latest Swift.
• Switching to Xcode 14 fixed the issues.

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[stale[bot]]

This issue has been closed due to inactivity.