Coupon Codes Controller: Use new extension point

[mamhoff]

Summary

The extension point for the coupon code handler class has been moved to the configurable promotions object. Removes a lot of deprecation warnings from the test suite.

Checklist

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.98%. Comparing base (6e2f055) to head (f3e5675).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #366 +/- ## ========================================== - Coverage 95.99% 95.98% -0.02% ========================================== Files 207 207 Lines 4874 4855 -19 ========================================== - Hits 4679 4660 -19 Misses 195 195 ```

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