Replaced macos-15-intel from macos-13

[hsbt]

We may test it only 2.5 and macos-15-intel. But we have enough credit to test fiddle gem with Intel macOS under the ruby organization.

[Copilot]

Pull request overview

This PR updates the CI workflow to replace the macos-13 runner with macos-15-intel, allowing the fiddle gem to be tested on Intel-based macOS 15 systems using available credits from the Ruby organization.

• Removes macos-13 from the test matrix
• Adds macos-15-intel to test on Intel-based macOS 15

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Missing exclusion for Ruby 2.5 with macos-15-intel. Lines 37-38 exclude Ruby 2.5 from both macos-14 and macos-15, but there's no corresponding exclusion for macos-15-intel. This inconsistency suggests that Ruby 2.5 will be tested on macos-15-intel but not on the other macOS 15+ runners. Consider adding an exclusion like:

- { os: macos-15-intel , ruby: '2.5' }

unless Ruby 2.5 support on Intel macOS 15 is intentional.