ci: align OS matrix with httpstan wheels

Author: riddell-stan

.github/workflows/main.yml

@@ -6,14 +6,11 @@ jobs:
   tests:
     name: pystan tests
     timeout-minutes: 30
-    runs-on: ${{ matrix.runs-on }}
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        include:
-          - {runs-on: ubuntu-24.04, python-version: "3.12"}
-          - {runs-on: ubuntu-24.04, python-version: "3.13"}
-          - {runs-on: ubuntu-24.04, python-version: "3.14"}
-          - {runs-on: macos-15-intel, python-version: "3.12"}
+        os: [ubuntu-24.04, macos-14]
+        python-version: ["3.12", "3.13", "3.14"]
     steps:
       - name: Check out repository
         uses: actions/checkout@v4

.github/workflows/publish.yaml

@@ -11,7 +11,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: "3.11"
+          python-version: "3.12"
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip