Downgrade MySQL CI service from 8.0 to 5.7

MySQL 8.0 made the default authentication mode `caching_sha2_password`, and
this breaks our CI builds at the moment. Downgrade until we can figure
out how to support this.

Closes #45606

.gitlab-ci.yml

@@ -75,7 +75,7 @@ stages:
 
 .use-mysql: &use-mysql
   services:
-    - mysql:latest
+    - mysql:5.7
     - redis:alpine
 
 .rails5-variables: &rails5-variables