Don't error when ENGINE_API_KEY and APOLLO_KEY are found

[JakeDawkins]

apollo-server still uses the ENGINE_API_KEY, so we can't truly move to the newer key. We have to be able to support both keys for now, so this removes the error and replaces it with a warning that explicitly states which key tooling is using.

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.

[zionts]

Works for me! I made a corresponding update to apollo server: apollographql/apollo-server@a83a1f5