Bump actions/checkout from 2.4.0 to 3

Bumps [actions/checkout](https://github.com/actions/checkout) from 2.4.0 to 3.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v2.4.0...v3)

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

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

.github/workflows/phpunit.yml

@@ -44,7 +44,7 @@ jobs:
 
     steps:
       - name: Checkout server
-        uses: actions/checkout@v2.4.0
+        uses: actions/checkout@v3
         with:
           repository: nextcloud/server
           ref: ${{ matrix.server-versions }}
@@ -57,7 +57,7 @@ jobs:
           git -c "http.extraheader=$auth_header" -c protocol.version=2 submodule update --init --force --recursive --depth=1
 
       - name: Checkout app
-        uses: actions/checkout@v2.4.0
+        uses: actions/checkout@v3
         with:
           path: apps/${{ env.APP_NAME }}