Fix Sprockets Dependency

[jarednorman]

Summary

Core itself doesn't care about the presence of sprockets. The starter frontend doesn't need it. I don't think the new admin needs it. I think the old backend does, as does the old frontend.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have localized any and all user-facing strings that I added to the source code.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.50%. Comparing base (1e26db3) to head (a269744).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6404   +/-   ##
=======================================
  Coverage   89.50%   89.50%           
=======================================
  Files         981      981           
  Lines       20479    20479           
=======================================
  Hits        18329    18329           
  Misses       2150     2150           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.