Replace `autoprefixer-rails` gem with direct use of `autoprefixer` npm dependency

The following discussion from !65385 (merged) should be addressed:

• @stanhu started a discussion: (+1 comment)

  @leipert I'm not quite sure why we have autoprefixer-rails and autoprefixer in yarn.lock, but I see you added this in !52101 (merged). Can you review? https://my.diffend.io/gems/autoprefixer-rails/10.2.0.0/10.2.5.1 is the full diff here.

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