Test on HAProxy 3.1

Author: TimWolla

.github/workflows/test.yml

@@ -49,6 +49,7 @@ jobs:
         - "2.8"
         - "2.9"
         - "3.0"
+        - "3.1"
       fail-fast: false
     steps:
     - name: Install HAProxy ${{ matrix.haproxy-versions }}.