Swap :unprocessable_entity to :unprocessable_content

Rack 3.1 changes the 422 constant to unprocessable_content, and the linter is picking up that we should change it.

However, we'd need to fix the minimum Rack version to 3.1 to do this. Rails 7 and 8 requires at least 2.2, so this would be a much tighter requirement than Rails itself.

I think it's fine to do so, but want to think about it first.