Don't consider existing tags when setting object tags

We recently changed how to set object tags, to always consider what exists before blindly setting them.

https://github.com/dandi/dandi-archive/blob/f222f03af8ddc1e628fcb0422cabb69ea1c927c7/dandiapi/api/services/embargo/utils.py#L26-L29

However, since we know what the value of these tags will be from the database, we should authoritatively set them based on those database values. At the moment, the only tag we use is `embargoed`. However, in the future, that could change.

Start at dandiapi/api/services/embargo/utils.py, especially lines 26-29, and trace how the database supplies the current object tag values. Confirm that tag updates use those authoritative values rather than considering existing tags; the current behavior should remain correct for the embargoed tag and future database-backed tags.

Written by the indexing model from the issue text.