Rails displays an error message if you have a bundler-generated binstub
at `./bin/rails` instead of a Rails-generated binstub.
This error message is misleading because it makes it seem as though
Rails 5 introduced recent changes in how binstubs are used, when these
changes were actually introduced way back in Rails 4.
The suggested upgrade steps are appropriate for an app that was created
in Rails 3, but they likely aren't the correct fix for someone who sees
this error message today on a modern app. I believe the `--binstubs`
option on bundler is a more likely culprit and troubleshooting path.
