Bump actions/checkout from 2 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v4)

---
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/crystal.yml

@@ -12,7 +12,7 @@ jobs:
     steps:
       - name: Install Crystal
         uses: crystal-lang/install-crystal@v1
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Install dependencies
         run: shards install
       - name: Check format
@@ -26,7 +26,7 @@ jobs:
       - name: Install Crystal
         uses: crystal-lang/install-crystal@v1
       - name: Check out repository code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
       - name: Install dependencies
         run: shards install
       - name: Run tests