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
```

model/utils.go

@@ -22,7 +22,7 @@ import (
 	"time"
 	"unicode"
 
-	"github.com/mattermost/mattermost-server/v5/shared/i18n"
+	"github.com/mattermost/mattermost-server/v6/shared/i18n"
 	"github.com/pborman/uuid"
 )