Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/cff-validator.yml

@@ -27,7 +27,7 @@ jobs:
     runs-on: "${{ matrix.os }}"
     steps:
       - name: "Checkout repository"
-        uses: "actions/checkout@v4"
+        uses: "actions/checkout@v6"
       - name: "Set up Python ${{ matrix.python-version }}"
         uses: "actions/setup-python@v5"
         with:

.github/workflows/codeql-analysis.yml

@@ -27,7 +27,7 @@ jobs:
       security-events: "write"
     steps:
       - name: "Checkout repository"
-        uses: "actions/checkout@v4"
+        uses: "actions/checkout@v6"
       - name: "Set up Python ${{ matrix.python-version }}"
         uses: "actions/setup-python@v5"
         with:

.github/workflows/tests.yml

@@ -37,7 +37,7 @@ jobs:
     runs-on: "${{ matrix.os }}"
     steps:
       - name: "Checkout repository"
-        uses: "actions/checkout@v4"
+        uses: "actions/checkout@v6"
       - name: "Set up Python ${{ matrix.python-version }}"
         uses: "actions/setup-python@v5"
         with:
@@ -176,7 +176,7 @@ jobs:
     runs-on: "${{ matrix.os }}"
     steps:
       - name: "Checkout repository"
-        uses: "actions/checkout@v4"
+        uses: "actions/checkout@v6"
       - name: "Set up Python ${{ matrix.python-version }}"
         uses: "actions/setup-python@v5"
         with: