Bump up the minimum required MySQL version (#25465)

MySQL 5.7 is at end of life.

https://mattermost.atlassian.net/browse/MM-55589

```release-note
We bump up minimum MySQL version to be 8.0.0
```


Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

server/build/gitlab-dc.mysql.yml

@@ -1,7 +1,7 @@
 version: '2.4'
 services:
   mysql:
-    image: mattermostdevelopment/mirrored-mysql:5.7.12
+    image: mattermostdevelopment/mirrored-mysql:8.0.32
     restart: always
     environment:
       MYSQL_ROOT_HOST: "%"