scalar-functional-tests: upgrade to a newer macOS pool

The macos-13 runner pool will soon be closing down, as per
https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down/

Unlike the Git project in 73b9cdb (GitHub CI: macos-13 images are no
more, 2025-11-04), let's not upgrade to the next-newer pool timidly, but
boldly go to the latest pool, macos-15.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/scalar-functional-tests.yml

@@ -20,7 +20,7 @@ jobs:
       fail-fast: false
       matrix:
         # Order by runtime (in descending order)
-        os: [windows-2022, macos-13, ubuntu-22.04]
+        os: [windows-2022, macos-15, ubuntu-22.04]
         # Scalar.NET used to be tested using `features: [false, experimental]`
         # But currently, Scalar/C ignores `feature.scalar` altogether, so let's
         # save some electrons and run only one of them...