CI: bump actions/checkout to v5 and setup-python to v6 (Node 24)

The v4/v5 (and v2 in tests.yml) majors run on Node 20, which GitHub
runners are deprecating in favor of Node 24.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: stonebig

.github/workflows/publish.yml

@@ -11,8 +11,8 @@ jobs:
     permissions:
       id-token: write    # OIDC : this replaces the PyPI token
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
+      - uses: actions/checkout@v5
+      - uses: actions/setup-python@v6
         with:
           python-version: "3.12"
       - run: |

.github/workflows/tests.yml

@@ -19,9 +19,9 @@ jobs:
         python-version: [3.14]
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v5
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v2
+      uses: actions/setup-python@v6
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies