Projects: remove `requirements_file` (#11132)
* Projects: remove old/non-used fields
Follow safe migrations instructions from
https://dev.readthedocs.io/en/latest/migrations.html to remove these fields.
* Lint
* Keep `Project.documentation_type` to avoid breaking changes
* Remove fields from fixture
* Remove more leftovers
* Lint
* Rename migrations
* Fix tests
* Projects: remove `requirements_file`
This field is not required anymore. It has to be defined in the YAML config file
now. This PR continues the work done in #11124
* Update migration
