Add pre-2023 map data set as an option

[pdil]

2023 resulted in a major change to the way Connecticut counties are defined, see https://www.ctinsider.com/projects/2023/ct-planning-regions/.

Several users have requested the ability to plot the US map before this change came into effect, since plotting older data sets is not compatible with the latest version of the map and results in Connecticut not filling in correctly.

See pdil/usmap#110 and pdil/usmap#111

More resources:

• 2020 announcement
• 2022 changes made

[pdil]

As part of this change, the update-map-data workflow will keep the prior year data sets instead of overwriting them. Very old data sets may be deleted periodically to prevent the file size of the package from growing too much.