Update macOS example config to Xcode 26.3.0 and m4pro.medium

[kevnm67]

Description

Updated the macOS hello-world example configuration file to use Xcode 26.3.0 and added resource_class: m4pro.medium to both the test and build jobs.

Reasons

The example config referenced Xcode 14.2.0, which is outdated. Updated to reflect current Xcode availability and added the m4pro.medium resource class to demonstrate recommended usage with Apple Silicon runners, helping contributors get accurate, working configs from the docs.

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