Bump actions/checkout from v1 to v2.3.1 (#588)

Bumps [actions/checkout](https://github.com/actions/checkout) from v1 to v2.3.1.
- [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/v1...28c7f3d2b5162b5ddd3dfd9a45aa55eaf396478b)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/maven.yml

@@ -31,7 +31,7 @@ jobs:
             experimental: true        
         
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2.3.1
     - name: Set up JDK ${{ matrix.java }}
       uses: actions/setup-java@v1
       with: