Bump actions/checkout from 5 to 6 (#8)

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...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] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/docker-publish.yml

@@ -23,7 +23,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       # Workaround: https://github.com/docker/build-push-action/issues/461
       - name: Setup Docker buildx

.github/workflows/lint.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Check out code
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
 
     - name: Set up python
       uses: actions/setup-python@v6

.github/workflows/test.yml

@@ -23,7 +23,7 @@ jobs:
           - tests/test_lm.py
           - tests/test_logging.py
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           repository: 'UCL/TLOmodel'
           lfs: false