ci: Replace unsupported macos-13 runner with macos-15

The macos-13 runner is unsupported since 2025-12-04
actions/runner-images#13046

Signed-off-by: Janne Grunau <j@jannau.net>

Author: jannau

.github/workflows/test.yml

@@ -18,10 +18,10 @@ jobs:
             compiler: gcc
           - os: ubuntu-latest
             compiler: llvm
-          - os: macos-13
-            compiler: llvm-brew
           - os: macos-14
             compiler: llvm-brew
+          - os: macos-15
+            compiler: llvm-brew
 
     steps:
       - name: Checkout code