CI: Run Ruby 2.5 on macos-13

[olleolleolle]

This follows the directions of the setup-ruby GitHub Action.

        If you are using a matrix of Ruby versions, a good solution is to run only < 2.6 on amd64, like so:
        matrix:
          ruby: ['2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3']
          os: [ ubuntu-latest, macos-latest ]
          # CRuby < 2.6 does not support macos-arm64, so test those on amd64 instead
          exclude:
          - { os: macos-latest, ruby: '2.5' }
          include:
          - { os: macos-13, ruby: '2.5' }

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