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

@@ -80,7 +80,7 @@ stages:
 
 .use-mysql: &use-mysql
   services:
-    - mysql:latest
+    - mysql:5.7
     - redis:alpine
 
 # BEGIN EE-only service helpers