bugfix of issue #373: handling of nested metadata fields with different visibility settings

[saschaszott]

This PR fixes the bug described in Github issue #373.

[abollini]

@saschaszott this cannot be considered for 2023.02.00 as, after further investigation, we found that is not appropriate to keep in version 7 the security at the level of each individual value of a nested metadata. We are going to enforce the UI to manage a single security value for a whole "row" of a nested group so that the data will be more consistent. If the need is to keep only part of information of a nested restricted this will be managed via the layout providing different layout where the same nested is shown with more or less details according to the logged in user.
This mean that the import script and your fix must be revisited to get the security level from the leading metadata of each group instead than from the individual metadata in the nested

[github-actions]

This pull request has been automatically marked as stale because it has not had
activity in one year. It will be closed in 14 days if no further activity occurs.

Allowing pull requests to close as stale helps us filter out old work that is no longer
relevant and helps developers focus on reviewing current work.

All pull requests closed by this bot act like normal pull requests;
they can be searched for, commented on or reopened at any point.

If these changes are still relevant then please comment and/or rebase your PR based on the
latest DSpace code. This will remove the stale status and notify us to assign a reviewer
for your PR.

[saschaszott]

Has the problem been fixed in the meantime?