Revert MySQL to 5.7.40.

This avoids connection problems when caching_sha2_password is the default authentication method.

Author: bgrainger

azure-pipelines.yml

@@ -1,7 +1,7 @@
 resources:
   containers:
   - container: mysql
-    image: mysql:8.0.31
+    image: mysql:5.7.40
     ports:
     - "3300:3306"
     env:

docker/docker-compose.yml

@@ -10,7 +10,7 @@ services:
       SA_PASSWORD: P@ssw0rd
 
   mysql:
-    image: mysql:8
+    image: mysql:5.7.40
     ports:
       - "3306:3306"
     environment: