Bump actions/checkout from v1 to v2.3.1 (#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](https://github.com/actions/checkout/compare/v1...28c7f3d2b5162b5ddd3dfd9a45aa55eaf396478b)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
diff --git a/.github/workflows/maven.yml b/.github/workflows/maven.yml
index 915dab6..58a7207 100644
--- a/.github/workflows/maven.yml
+++ b/.github/workflows/maven.yml
@@ -26,7 +26,7 @@
         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: