Add explicit responders dependency

[kennyadsl]

Summary

With solidusio/solidus#5158, we removed respond_to :html from Spree::BaseController. That's because responders is no more a core dependency and Spree::BaseController is defined there.

Still, the starter frontend is using it and we can't remove all occurrences of its usage now, mainly because part of them are used in the authentication system (it's a devise dependency) and it takes some time to understand what we can remove/change without security issues.

We take advantage of this change to also remove some useless usage of respond_with.

Checklist

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[codecov[bot]]

Codecov Report

Merging #347 (f3e1726) into main (eaadb0b) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #347   +/-   ##
=======================================
  Coverage   97.13%   97.13%           
=======================================
  Files         190      190           
  Lines        4500     4501    +1     
=======================================
+ Hits         4371     4372    +1     
  Misses        129      129           

Impacted Files	Coverage Δ
template.rb	85.13% <100.00%> (+0.20%)	:arrow_up:
...ates/app/controllers/cart_line_items_controller.rb	95.65% <100.00%> (ø)
templates/app/controllers/carts_controller.rb	90.24% <100.00%> (ø)
...mplates/app/controllers/coupon_codes_controller.rb	100.00% <100.00%> (ø)

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