Drop autoprefixer-rails from dependencies

[elia]

Summary

No need to force it on the whole application asset pipeline in 2023, and it's easy to add it back locally if really needed.

Also gets rid of that annoying message about color-adjust:

autoprefixer: /Users/elia/Code/Nebulab/solidus_stripe/dummy-app/vendor/assets/stylesheets/spree/backend/all.css:1992:5: Replace color-adjust to print-color-adjust. The color-adjust shorthand is currently deprecated.

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