Enable `--warn-redundant-casts` option in mypy (#14671)

* Enable `--warn-redundant-casts` option in mypy

Doesn't do much but helps me sleep better at night.

* Changelog

* Fix name of the ignore

* Fix one more missed cast

Not sure why I didn't see this one locally, maybe I needed a poetry update

* Remove old comment

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>