use MySQL 8.0 in CI

Author: driehle

.github/workflows/continuous-integration.yml

@@ -48,7 +48,7 @@ jobs:
 
     services:
       mysql:
-        image: "mysql:5.7"
+        image: "mysql:8.0"
 
         options: >-
           --health-cmd "mysqladmin ping --silent"