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] <[email protected]>

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
     name: elixir-${{ matrix.pair.elixir }}-otp-${{ matrix.pair.otp }}
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - uses: erlef/setup-elixir@v1
       with: