heroku: don't suggest setting both node and npm version (#3739)
heroku: don't suggest setting both node and npm version (#3739)

per discussion in https://github.com/phoenixframework/phoenix/pull/3706

setting both node and npm - will lead to issues down the line (eg when user wants to use node 12 etc) - it's also complicated figuring out what npm version to actually use etc. etc.

we are still conservative on suggesting node 10 - could be future proofing going 12.16.1.. but that is separate issue..
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