Update @oclif/plugin-warn-if-update-available to 3.x

[chillu]

The latest 10.x beta release is still using an old 2.x release line of @oclif/plugin-warn-if-update-available. The 3.0 release has been out for over a year.

The reason I'm asking for this upgrade is that I want to use the SKIP_UPDATE_CHECK env var to avoid heroku delaying my builds with update checks, through the scoped HEROKU_SKIP_UPDATE_CHECK (see pull request)

[k80bowman]

Thank you for the request, we'll look into it.

Internal work item