MM-36271: Bump major version to 6.0 (#17973)

https://mattermost.atlassian.net/browse/MM-36271

```release-note
We bump the major version to 6.0
```

config/database.go

@@ -17,8 +17,8 @@ import (
 	// Load the Postgres driver
 	_ "github.com/lib/pq"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 // MaxWriteLength defines the maximum length accepted for write to the Configurations or