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
```

api4/user_local.go

@@ -7,9 +7,9 @@ import (
 	"net/http"
 	"strconv"
 
-	"github.com/mattermost/mattermost-server/v5/audit"
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/store"
+	"github.com/mattermost/mattermost-server/v6/audit"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/store"
 )
 
 func (api *API) InitUserLocal() {