Merge branch 'sh-downgrade-mysql-ci' into 'master'

Downgrade MySQL CI service from 8.0 to 5.7

Closes #45606

See merge request gitlab-org/gitlab-ce!18512

.gitlab-ci.yml

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