Update fullstack tutorial to use Apollo Server 3

[StephenBarlow]

No description provided.

[glasser]

There are some missing changes in index.js (they would be errors if this were in TypeScript... maybe it should be :) )

• remove playground: true
• i don't really understand what the heck "internalEngineDemo" is. What do we need that for? But it should be changed to not have the word "engine" in it (or remove it?), and it should have a graphVariant field instead of schemaTag (though current is also the default so 🤷 ). And honestly, the generateClientInfo there looks weird too — it's looking for old header names that don't start with apollographql-. So unless the tutorials ask you to set exactly those headers, just remove this whole file? I suspect this is so old that it predates client info being gathered by default.