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
```

services/httpservice/httpservice.go

@@ -10,7 +10,7 @@ import (
 	"time"
 	"unicode"
 
-	"github.com/mattermost/mattermost-server/v5/services/configservice"
+	"github.com/mattermost/mattermost-server/v6/services/configservice"
 )
 
 // HTTPService wraps the functionality for making http requests to provide some improvements to the default client