[MNG-7537] Point to the old IT branch when building 3.8.x (#799)

.github/workflows/maven.yml

@@ -64,7 +64,7 @@ jobs:
         run: |
           set +e
           repo=maven-integration-testing
-          target_branch=master
+          target_branch=maven-3.8.x
           target_user=apache
           if [ "$GITHUB_EVENT_NAME" == "pull_request" ]; then
             user=${PR_HEAD_LABEL%:*}