Add CODEOWNERS for migrations (#23020)

```release-note
NONE
```

CODEOWNERS

@@ -5,3 +5,6 @@
 /webapp/package-lock.json @mattermost/web-platform
 /webapp/platform/*/package.json @mattermost/web-platform
 /webapp/scripts @mattermost/web-platform
+/server/channels/db/migrations @mattermost/server-platform
+/server/boards/services/store/sqlstore/migrations @mattermost/server-platform
+/server/playbooks/server/sqlstore/migrations @mattermost/server-platform