Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/publishing.yaml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: astral-sh/setup-uv@v7
       - name: Setup Python
         run: uv python install 3.14

.github/workflows/testing.yaml

@@ -37,7 +37,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: astral-sh/setup-uv@v7
       - name: Setup Python
         run: uv python install ${{ matrix.python-version }}