ci: upgrade GitHub Actions to Node 24-compatible versions

GitHub is deprecating Node.js 20 on Actions runners (default switches to
Node 24 on 2026-06-16, full removal 2026-09-16). Bump the affected actions
(actions/checkout, actions/setup-python, and peter-evans/create-pull-request
where used) to versions that ship Node 24 support.

Part of overhangio/tutor#1398

Author: Abdul-Muqadim-Arbisoft

.github/workflows/sync.yml

@@ -8,7 +8,7 @@ jobs:
   sync:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - name: Add remote

.github/workflows/test.yml

@@ -13,9 +13,9 @@ jobs:
       matrix:
         python-version: ["3.9", "3.12"]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set up ${{ matrix.python-version }}
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@v6
         with:
           python-version: ${{ matrix.python-version }}
       - name: Install dependencies