`date_modified` on datasets is set incorrectly

[mohammad-alisafaee]

Describe the bug date_modified property on the datasets is always set to current date/time which is not correct: https://github.com/SwissDataScienceCenter/renku-python/blob/add433e3b52854b5546bf9c57a09e893c3ff0d7b/renku/domain_model/dataset.py#L367 A manifestation of this bug is visible in renku log.

To Reproduce

1. renku clone [email protected]:rok.roskar/atmospheric-vertical-profiles.git
2. renku log
3. See Date on each entry

Expected behavior

• date_modified should get its value from the persisted value
• migration must set proper date_modified on the datasets; we can use date_created or commit date/time
• current values for this field should be fixed either by a renku doctor fix or by using date_created