gha: add macOS 15, remove macOS 13 (deprecated)

The macOS 13 runners are deprecated and will be removed on December 4th,
with brownouts in November;
https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down/

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah

.github/workflows/test.yml

@@ -53,8 +53,9 @@ jobs:
       fail-fast: false
       matrix:
         os:
-          - macos-13  # macOS 13 on Intel
-          - macos-14  # macOS 14 on arm64 (Apple Silicon M1)
+          - macos-14        # macOS 14 on arm64 (Apple Silicon M1)
+          - macos-15-intel  # macOS 15 on Intel
+          - macos-15        # macOS 15 on arm64 (Apple Silicon M1)
 #          - windows-2022 # FIXME: some tests are failing on the Windows runner, as well as on Appveyor since June 24, 2018: https://ci.appveyor.com/project/docker/cli/history
     steps:
       -