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/user.go

@@ -18,8 +18,8 @@ import (
 	"golang.org/x/crypto/bcrypt"
 	"golang.org/x/text/language"
 
-	"github.com/mattermost/mattermost-server/v5/services/timezones"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/services/timezones"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 const (