Decorate Default Return Refund Amount Calculator in LegacyPromotions

[mamhoff]

This calculator uses the eligible API from Spree::Adjustment, which in a system with the new promotions system will not be necessary.

This hasn't been caught before because deprecation warnings did not raise until #5813 was merged.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.17%. Comparing base (1aa18ee) to head (f739843).
Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
...culator_returns_default_refund_amount_decorator.rb	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5845      +/-   ##
==========================================
+ Coverage   89.14%   89.17%   +0.03%     
==========================================
  Files         744      749       +5     
  Lines       17343    17395      +52     
==========================================
+ Hits        15460    15512      +52     
  Misses       1883     1883              

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[MadelineCollier]

Thanks! :)