Fix CI for Ruby 2.5

[qaisjp]

Following the guidance from the build failure:

Error: Error: CRuby < 2.6 does not support macos-arm64.
        Either use a newer Ruby version or use a macOS image running on amd64, e.g., macos-13.
        Note that GitHub changed the meaning of macos-latest from macos-12 (amd64) to macos-14 (arm64):
        https://github.blog/changelog/2024-04-01-macos-14-sonoma-is-generally-available-and-the-latest-macos-runner-image/

        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: '2.4' }
          - { os: macos-latest, ruby: '2.5' }
          include:
          - { os: macos-13, ruby: '2.4' }
          - { os: macos-13, ruby: '2.5' }

        But of course you should consider dropping support for these long-EOL Rubies, which cannot even be built on recent macOS machines.