[BUG] Asset 'date_update' field is not updated after asset update

[StaSys846]

Describe the bug
The date_update field of a case asset is not updated after asset update. This applies to versions of IRIS <=2.4.14

To Reproduce
Steps to reproduce the behavior:

1. Create a new case asset and save the asset
2. Re-open newly created asset and make modifications
3. Save the modified asset
4. date_update field of the asset is not updated and remains at the creation date

Expected behavior
Any asset modification should result in an update of the date_update field to the current modification storage time.

Additional context
The field name date_update implies, that the field should be updated with the current time when the asset object is modified/updated.
date_update field is set once during asset create, but never touched again in the code afterwards.

• date_update and date_added are set correctly during asset create (see source/app/datamgmt/case/case_assets_db.py line 46/47).
• date_update is not updated in the update routine (see source/app/datamgmt/case/case_assets_db.py line 105ff)