Update generator data in HIFLD grid-building to latest

# :rocket:

- [ ] Is your feature request essential for your project?

### Describe the workflow you want to enable
Update EIA generator dataset to latest available for building HIFLD grid.

### Describe your proposed implementation, if applicable

No code change required (I hope). We're currently hosting and using the 2019 EIA Form 860 data, but the final 2020 data are available now (since last September) and the 'early release' 2021 data should be available sometime next month: https://www.eia.gov/electricity/data/eia860/.

Similarly, we could also update the EPA CEMS data to the most recent full year of data, and/or have multiple years of data available to use for curve-fitting.