Update deprecation message of ENGINE_API_KEY to include alternative of APOLLO_KEY

[zionts]

The message that appeared for users using ENGINE_API_KEY was not actionable before this change. This updates the message to suggest using APOLLO_KEY in lieu of ENGINE_API_KEY :)

I don't think this kind of change really necessitates a CHANGELOG entry, but happy to add one if requested.

TODO:

• 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.

[JakeDawkins]

Thanks @zionts!