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](actions/checkout@ec3a7ce...a12a394)

---
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/build.yml

@@ -15,7 +15,7 @@ jobs:
             java: [ '11', '11.0.3', '15', '15.0.5']
     name: Test Java ${{ matrix.Java }}, ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@ec3a7ce113134d7a93b817d10a8272cb61118579
+    - uses: actions/checkout@a12a3943b4bdde767164f792f33f40b04645d846
       with:
         fetch-depth: 0
         ref: ${{ github.event.pull_request.head.sha }}