Fix migration scripts (#18756)

* Fix migration scripts

Set the DB schema version in DB prior to running migration

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4?c=c2525dbb-23eb-4fe7-8fb4-e9a5f503dc8c

```release-note
NONE
```

* Revert schema to 6.0.0 and update CI scripts

* Fix typo

* Add missing default

* Fix extra comma

Co-authored-by: Claudio Costa <cstcld91@gmail.com>

store/sqlstore/channel_store.go

@@ -386,6 +386,7 @@ func newSqlChannelStore(sqlStore *SqlStore, metrics einterfaces.MetricsInterface
 		table.ColMap("Purpose").SetMaxSize(250)
 		table.ColMap("CreatorId").SetMaxSize(26)
 		table.ColMap("SchemeId").SetMaxSize(26)
+		table.ColMap("LastRootPostAt").SetDefaultConstraint(model.NewString("0"))
 
 		tablem := db.AddTableWithName(channelMember{}, "ChannelMembers").SetKeys(false, "ChannelId", "UserId")
 		tablem.ColMap("ChannelId").SetMaxSize(26)