Upgrade CI from macos-15-intel to macos-26-intel

[hugovk]

macOS 26 is now available on GitHub Actions:

• macos-26 - arm64
• macos-26-intel - x64

https://github.blog/changelog/2026-02-26-macos-26-is-now-generally-available-for-github-hosted-runners/

This PR bumps macos-15-intel to macos-26-intel.

I've left macos-latest as-is. They're still pointing to macOS 15, but that will change in the near future.

[radarhere]

This is the last time that we will need to upgrade like this, as https://en.wikipedia.org/wiki/MacOS_Tahoe

Tahoe is the final version of macOS that supports Macs with Intel processors

[hugovk]

Yes.

Tahoe is EOL in ~3 years, but GitHub's N-1 support policy:

• We support (at maximum) 2 GA images and 1 beta image at a time. We begin the deprecation process of the oldest image label once the newest OS image label has been released to GA.

So macos-26-intel will be around for ~2 years.

This could of course change, before they introduced the *-intel runners, macos-13 was to be the last Intel.