Improve error messaging when installing an incompatible npm version

[joshwlewis]

When installing npm@10 with node@16, users are seeing error messages like this: Unable to install npm >=9; does it exist?.

The issue here is that npm@10 has minimum requirements for the Node.js version it works with: https://github.com/npm/cli/blob/3f9aa452061cde22de1babb33fd9bed8f2b6e3a5/package.json#L257

The npm command emits an error message explaining the issue that looks like this:

npm ERR! code EBADENGINE
npm ERR! engine Unsupported engine
npm ERR! engine Not compatible with your version of node/npm: [email protected]
npm ERR! notsup Not compatible with your version of node/npm: [email protected]
npm ERR! notsup Required: {"node":"^18.17.0 || >=20.5.0"}
npm ERR! notsup Actual:   {"npm":"8.5.0","node":"v16.14.0"}

However, due to the use of shell redirection in the buildpack, the error message never makes it to the user.

This PR drops the stderr redirection so that the underlying error message does appear. In order prevent other needless output on stderr, --no-progress and --no-audit have been added to the install command.

I've also added another hint to the final error message about compatibility.

Related:
KB article explaining the situation and a solution
GUS Work Item

[edmorley]

@joshwlewis Is this ready to merge?