Bump actions/checkout from 2.3.4 to 2.4.0

Bumps [actions/checkout](https://github.com/actions/checkout) from 2.3.4 to 2.4.0.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2.3.4...v2.4.0)

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

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

.github/workflows/CI.yml

@@ -15,7 +15,7 @@ jobs:
         dependencies: ['', '--prefer-lowest --prefer-stable']
     name: Tests on PHP ${{ matrix.php-versions }} ${{ matrix.dependencies }}
     steps:
-      - uses: actions/checkout@v2.3.4
+      - uses: actions/checkout@v2.4.0
       - name: Checkout submodules
         shell: bash
         run: |
@@ -42,7 +42,7 @@ jobs:
     runs-on: ubuntu-18.04
     name: Code quality
     steps:
-      - uses: actions/checkout@v2.3.4
+      - uses: actions/checkout@v2.4.0
       - uses: shivammathur/setup-php@2.16.0
         with:
           php-version: '7.4'