chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/check-removed-urls.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout PR branch
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           # This implicitly gets the PR branch. Making it explicit causes problems
           # with private forks, but it is equivalent to the following:
@@ -19,7 +19,7 @@ jobs:
           fetch-depth: 0
           path: compare
       - name: Checkout base branch
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           ref: ${{ github.event.pull_request.base.ref }}
           repository: ${{ github.event.pull_request.base.repo.full_name }}

.github/workflows/markdown-style-checks.yml

@@ -17,7 +17,7 @@ jobs:
   markdown-lint:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
     - name: Create venv

.github/workflows/sphinx-python-dependency-build-checks.yml

@@ -28,7 +28,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout code
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
 
     - name: Install dependencies
       run: |