upgrade actions/checkout to v3

Author: JacksonTian

.github/workflows/maven.yml

@@ -21,7 +21,7 @@ jobs:
         java: [ 8, 11 ]
     name: Java ${{ matrix.Java }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Setup java
       uses: actions/setup-java@v2
       with: