[legacy_promotions]: Correct decorator naming

[mamhoff]

Summary

This changes the file names of decorators in the solidus_legacy_promotions gem to match Zeitwerk's expectations. solidus_promotions is actually entirely fine.

This has been tested with https://github.com/solidusio/solidus_support/pull/87, but is fully compatible with the classic way of loading each and every decorator on every app reload.

Checklist

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The following are mandatory for all PRs:

• [x] I have written a thorough PR description.
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[codecov[bot]]

Codecov Report

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

Project coverage is 89.45%. Comparing base (104f813) to head (fe18ce3).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5946 +/- ## ========================================== - Coverage 89.54% 89.45% -0.09% ========================================== Files 782 782 Lines 17997 17997 ========================================== - Hits 16116 16100 -16 - Misses 1881 1897 +16 ```

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