Bump actions/checkout from 2.3.5 to 3.1.0

Bumps [actions/checkout](https://github.com/actions/checkout) from 2.3.5 to 3.1.0.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2.3.5...v3.1.0)

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

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

Author: dependabot[bot]

.github/workflows/elixir.yml

@@ -27,7 +27,7 @@ jobs:
           POSTGRES_USER: postgres
         options: --health-cmd pg_isready --health-interval 10s --health-timeout 5s --health-retries 3
     steps:
-      - uses: actions/checkout@v2.3.5
+      - uses: actions/checkout@v3.1.0
       - uses: erlef/setup-elixir@v1.14
         with:
           otp-version: ${{matrix.otp}}