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>

model/channel.go

@@ -47,7 +47,6 @@ type Channel struct {
 	Header            string                 `json:"header"`
 	Purpose           string                 `json:"purpose"`
 	LastPostAt        int64                  `json:"last_post_at"`
-	LastRootPostAt    int64                  `json:"last_root_post_at"`
 	TotalMsgCount     int64                  `json:"total_msg_count"`
 	ExtraUpdateAt     int64                  `json:"extra_update_at"`
 	CreatorId         string                 `json:"creator_id"`
@@ -57,6 +56,7 @@ type Channel struct {
 	Shared            *bool                  `json:"shared"`
 	TotalMsgCountRoot int64                  `json:"total_msg_count_root"`
 	PolicyID          *string                `json:"policy_id" db:"-"`
+	LastRootPostAt    int64                  `json:"last_root_post_at"`
 }
 
 type ChannelWithTeamData struct {