build: bump from macOS 12 to macOS 13

[ee7]

macOS 13 is available in GitHub-hosted runners since 2023-04-24.

At the time of writing, it's still considered beta, and macos-latest uses macOS 12.

Related: https://github.com/exercism/configlet/issues/661

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Their defintion of beta:

Image Definitions

Beta

The purpose of a Beta is to collect feedback on an image before it is released to GA. The goal of a Beta is to identify and fix any potential issues that exist on that image. Images are updated on a weekly cadence. Any workflows that run on a beta image do not fall under the customer SLA in place for Actions. Customers choosing to use Beta images are encouraged to provide feedback in the runner-images repo by creating an issue.