Support for Propshaft

[nickcharlton]

I've been working on Rails 8.0 support in #2705.

Rails 8.0 ships Propshaft as the default way to deliver assets in Rails, but Administrate assumes you're using sprockets, which has been the default for a long time. Propshaft has an upgrading guide which should be helpful.

I've heard of at least one report of a new thoughtbot client project (started this week or last) that's using Rails 8.0, using defaults with Propshaft has had a tricky time in getting Administrate up and running, but there's potentially more incompatibilities we should test with.

It'd be preferable if Administrate could support both options.

Related: #2514