ci: upgrade GitHub Actions to v6

Apply changes from PR #367 by @Copilot:
- actions/checkout v2 -> v6
- actions/setup-python v2 -> v6

.github/workflows/main.yml

@@ -10,9 +10,9 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
       - name: Set up Python
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v6
         with:
           python-version: '3.11'
       - name: Install uv

.github/workflows/manual_publish.yml

@@ -7,9 +7,9 @@ jobs:
   build-and-publish:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
       - name: Set up Python
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v6
         with:
           python-version: '3.11'
       - name: Install uv