chore: use maven 3.8.1 (#463)

Author: chingor13

.github/workflows/downstream.yaml

@@ -19,6 +19,9 @@ jobs:
           - java-pubsub
     steps:
       - uses: actions/checkout@v2
+      - uses: stCarolas/setup-maven@v4
+        with:
+          maven-version: 3.8.1
       - uses: actions/setup-java@v1
         with:
           java-version: ${{matrix.java}}