Add solidus legacy promotions to promotions gemfile

[mamhoff]

Summary

This adds solidus_legacy_promotions to the :promotions group in the main Gemfile, so that rake tasks in solidus_legacy_promotions run in development mode.

It also removes some duplication in the main Gemfile, removing these warnings:

Your Gemfile lists the gem solidus_admin (>= 0) more than once.
You should probably keep only one of them.
Remove any duplicate entries and specify the gem only once.
While it's not a problem now, it could cause errors if you change the version of one of them later.
Your Gemfile lists the gem axe-core-rspec (~> 4.8) more than once.
You should probably keep only one of them.
Remove any duplicate entries and specify the gem only once.
While it's not a problem now, it could cause errors if you change the version of one of them later.
Your Gemfile lists the gem axe-core-capybara (~> 4.8) more than once.
You should probably keep only one of them.
Remove any duplicate entries and specify the gem only once.
While it's not a problem now, it could cause errors if you change the version of one of them later.
Your Gemfile lists the gem solidus_legacy_promotions (>= 0) more than once.
You should probably keep only one of them.
Remove any duplicate entries and specify the gem only once.
While it's not a problem now, it could cause errors if you change the version of one of them later.
Your Gemfile lists the gem solidus_backend (>= 0) more than once.
You should probably keep only one of them.
Remove any duplicate entries and specify the gem only once.
While it's not a problem now, it could cause errors if you change the version of one of them later.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.82%. Comparing base (2d5bda7) to head (8849ae3).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6030      +/-   ##
==========================================
- Coverage   89.51%   87.82%   -1.70%     
==========================================
  Files         787      477     -310     
  Lines       18120    11670    -6450     
==========================================
- Hits        16220    10249    -5971     
+ Misses       1900     1421     -479     

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[kennyadsl]

Left a non-blocking question on future plans.