Revert "github_runner_matrix: enable usage of macos-15 runners"

[cho-m]

This reverts commit 6acce77.

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Xcode CLT doesn't seem to be properly set up on this runner and causes CI failures when pouring python@3.12.

e.g. https://github.com/Homebrew/homebrew-core/actions/runs/11038617543/job/30663276233?pr=191866#step:3:1001

  Error: python@3.12: the bottle needs the Apple Command Line Tools to be installed.
    You can install them, if desired, with:
      xcode-select --install

  Image: macos-15-arm64
  Version: 20240921.108
  Included Software: https://github.com/actions/runner-images/blob/macos-15-arm64/20240921.108/images/macos/macos-15-arm64-Readme.md
  Image Release: https://github.com/actions/runner-images/releases/tag/macos-15-arm64%2F20240921.108

CPU: 3-core 64-bit dunno
Clang: 16.0.0 build 1600
Git: 2.39.5 => /usr/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 15.0-arm64
CLT: N/A
Xcode: 16.0 => /Applications/Xcode_16.app/Contents/Developer
Rosetta 2: false

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• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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