Update shipit benchmark dependencies

- Upgrade to Rails 8.1
- Get rid of the dependency on sassc.

Author: byroot

benchmarks/shipit/Gemfile

@@ -1,7 +1,8 @@
 source "https://rubygems.org"
 
 # Bundle edge Rails instead: gem "rails", github: "rails/rails", branch: "main"
-gem "rails", "~> 8.0.2"
+gem "rails", "~> 8.1"
+gem "sprockets-rails"
 # Use sqlite3 as the database for Active Record
 gem "sqlite3", ">= 2.1"
 # Use the Puma web server [https://github.com/puma/puma]