CI: add macos-15-intel, remove deprecated macos-13

Author: mgeier

.github/workflows/tests.yml

@@ -13,7 +13,7 @@ jobs:
   test-bot:
     strategy:
       matrix:
-        os: [macos-13, macos-14, macos-15]
+        os: [macos-14, macos-15, macos-15-intel]
     runs-on: ${{ matrix.os }}
     steps:
       - name: Set up Homebrew