[Admin] Shipping Categories edit/update

[MadelineCollier]

Summary

This PR finishes off the excellent work by Loic that was left in draft on #5723
and includes an updated spec.

This PR follows the work started on #5718 where Shipping categories creation was implemented. This PR adds the edit/update functionality for shipping categories.
The issue is #5720

The attached video shows the functionality visually:

Screen.Recording.2024-08-08.at.7.14.00.PM.mov

Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 6 lines in your changes missing coverage. Please review.

Project coverage is 88.78%. Comparing base (3e8be72) to head (9398e80).

Files	Patch %	Lines
...rs/solidus_admin/shipping_categories_controller.rb	70.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5817      +/-   ##
==========================================
- Coverage   88.80%   88.78%   -0.02%     
==========================================
  Files         732      733       +1     
  Lines       17088    17114      +26     
==========================================
+ Hits        15175    15195      +20     
- Misses       1913     1919       +6     

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