* Upgrade mysql to 8.0.23 for github actions
* Fix thumbnail_url migration to work on Django 3 / MySQL 8.0.13+
Work around https://code.djangoproject.com/ticket/32503 by applying the not-null constraint before converting to TextField.
This should result in the docs workflow only running if files in the
`docs/` folder have changed, and only run the test workflow if files NOT
in the `docs/` folder have changed.
This commit itself should therefore, I think, only trigger the test
workflow. I will follow up with test commits for the other scenarios
(docs-only and both).