Upgrade to Rails 8

[mateusdeap]

This PR does two things:

• Upgrade to Rails 8.0
• Remove yarn

Removing yarn was merely because we have no JS packages that need to be pulled in nor compiled so it seemed to make sense to just not have that in the setup nor in the project at all.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.69%. Comparing base (9067188) to head (9c49eea).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #126   +/-   ##
=======================================
  Coverage   62.69%   62.69%           
=======================================
  Files          48       48           
  Lines         689      689           
=======================================
  Hits          432      432           
  Misses        257      257           

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