Add User Modification as "System Column"

[cancan101]

"User Modification" is not currently offered as a system column for Objects:

that field is on all Elements:
https://github.com/pimcore/pimcore/blob/15080bb27db59ce30680e5054bb2262ed41b1bbe/models/Element/AbstractElement.php#L102

[github-actions]

Thanks a lot for reporting the issue. We did not consider the issue as "Pimcore:Priority", "Pimcore:ToDo" or "Pimcore:Backlog", so we're not going to work on that anytime soon. Please create a pull request to fix the issue if this is a bug report. We'll then review it as quickly as possible. If you're interested in contributing a feature, please contact us first here before creating a pull request. We'll then decide whether we'd accept it or not. Thanks for your understanding.