Fix Intel-based macOS build

[AvishKaushik]

Summary

Fixes the Intel-based macOS build failures by upgrading from the deprecated macos-13 runner to the new macos-15-intel runner.

Problem

The Intel-based macOS build was disabled due to build failures. Investigation revealed that the macos-13 runner has been deprecated by GitHub Actions and is no longer receiving updates, causing the build to fail.

Solution

• Updated the Intel Mac runner from macos-13 to macos-15-intel
• Re-enabled the Intel-based macOS build that was temporarily disabled
• macos-15-intel is the official replacement for Intel-based macOS builds and will be supported until August 2027

Testing

All CI builds are now passing:

• Ubuntu (Python 3.10.15, 3.11)
• macOS ARM (macos-latest)
• macOS Intel (macos-15-intel)

Test run: https://github.com/AvishKaushik/sbsim/actions/runs/20201182727

References

• GitHub Actions: macOS 13 runner image is closing down
• macOS 15 Intel-based image availability

Resolves #126

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[s2t2]

@AvishKaushik thank you so much!! The Intel Mac build is back! 💪