Update readme to reflect `bin/dev` instead of `rails s` on bare metal

[colinxfleming]

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• What are we trying to do?

bump readme since the server boot commands have changed

• What feature or behavior is this required for?

• How could we solve this issue? (Not knowing is okay!)

• Anything else?
  see ditch webpacker in favor of jsbundling/esbuild #2594