⬆️ Upgrade actions/checkout action to v6 (#50)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/publish.yaml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     environment: pypi
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: true
 

.github/workflows/quality.yaml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Setup Copywrite
         uses: hashicorp/setup-copywrite@32638da2d4e81d56a0764aa1547882fc4d209636 # v1.1.3
       - name: Check Header Compliance
@@ -35,7 +35,7 @@ jobs:
             name: "Python 3.14"
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           submodules: true
 
@@ -75,7 +75,7 @@ jobs:
     name: ${{ matrix.name }}
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: "Install uv"
         uses: astral-sh/setup-uv@v7