Use `onupdate` for `last_updated_at`

[michplunkett]

Fixes issue

https://github.com/lucyparsons/OpenOversight/issues/1061

Description of Changes

Instead of manually updating last_updated_at, we should let SQL Alchemy take care of it.

Tests and linting

• [x] This branch is up-to-date with the develop branch.
• [x] pytest passes on my local development environment.
• [x] pre-commit passes on my local development environment.