[Admin] Fix rubocop offence and resources controller cleanup

[chaimann]

Summary

Fixes rubocop offence with duplicate methods, and creates default methods that should be overridden in child classes.

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 used clear, explanatory commit messages.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.86%. Comparing base (e6a57c3) to head (a0aca43).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
.../controllers/solidus_admin/resources_controller.rb	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6276      +/-   ##
==========================================
- Coverage   88.87%   88.86%   -0.01%     
==========================================
  Files         858      858              
  Lines       18413    18416       +3     
==========================================
+ Hits        16364    16366       +2     
- Misses       2049     2050       +1     

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