Switch from sprockets to propshaft for asset pipeline

[jrochkind]

Sprockets is now kind of abandonware. Rails wants propshaft instead. Although sprockets will keep working with no specific deadline.

As we have already switched all of our CSS and JS compiling/bundling from sprockets to vite, we've got the pre-reqs done, and this should be fairly straightforward to do! (famous last words, there still could be odd problems)

https://github.com/rails/propshaft

https://github.com/rails/propshaft/blob/main/UPGRADING.md

https://world.hey.com/dhh/introducing-propshaft-ee60f4f6