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/ci.yml

@@ -16,7 +16,7 @@ jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Install dependencies
         run: |
           python3 -m pip install --break-system-packages -q yamllint

.github/workflows/website.yml

@@ -42,13 +42,13 @@ jobs:
           fi
         shell: bash
       - name: Clone source repository on the previously computed branch
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           repository: multiphenics/multiphenicsx
           ref: ${{ steps.source_branch.outputs.branch }}
           fetch-depth: 0
       - name: Clone website repository on current branch
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
           path: _website
@@ -118,11 +118,11 @@ jobs:
     needs: [convert_notebooks]
     steps:
       - name: Clone website repository on current branch
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - name: Clone website repository on gh-pages branch
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           ref: gh-pages
           fetch-depth: 0