Add support for 2021 model

Valuations has said that 2021 model workbooks would be useful, so let's update our [list of supported models](https://github.com/ccao-data/api-res-avm/blob/36b21d471d0294389a0c4c1011162f5f257bea4f/api.R#L35-L61) to include it. This will probably require refactoring the code somewhat to account for differences between the 2021 and 2022 models.