Add new colors for new clades

[corneliusroemer]

The new clades 23F and 23G have just percolated, they are now missing colors:

[joverlee521]

Resolved by https://github.com/nextstrain/forecasts-ncov/commit/31a6163d76254ea53cd8def4e19ff6b5f8c77176.

New colors are showing up on https://nextstrain.org/sars-cov-2/forecasts