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/main.yml

@@ -13,15 +13,15 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: opensafely-core/research-action@v2
 
   tag-new-version:
     needs: [test-integration]
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Tag new version
         uses: mathieudutour/github-tag-action@a22cf08638b34d5badda920f9daf6e72c477b07b
         with:

.github/workflows/test_runner.yaml

@@ -12,6 +12,6 @@ jobs:
     name: Test the project can run, using dummy data
     steps:
     - name: Checkout
-      uses: actions/checkout@v4
+      uses: actions/checkout@v6
     - name: Test that the project is runnable
       uses: opensafely-core/research-action@v2