Encourage COMPOSER_AUTH over COMPOSER_GITHUB_OAUTH_TOKEN

[schneems]

The CNB does not support COMPOSER_GITHUB_OAUTH_TOKEN as it's from before COMPOSER_AUTH existed. To provide a smooth migration we should add a warning or suggestion that people use COMPOSER_AUTH if they're still using COMPOSER_GITHUB_OAUTH_TOKEN.

heroku-buildpack-php/bin/compile

Line 653 in 86d8503

COMPOSER_GITHUB_OAUTH_TOKEN=${COMPOSER_GITHUB_OAUTH_TOKEN:-}