Bump actions/checkout from v1 to v2.3.1 (apache#40)

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](actions/checkout@v1...28c7f3d)

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Author: dependabot[bot]

.github/workflows/maven.yml

@@ -26,7 +26,7 @@ jobs:
         java: [ 8, 9, 10, 11, 12, 13, 14 ]
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2.3.1
     - name: Set up JDK ${{ matrix.java }}
       uses: actions/setup-java@v1
       with: