Allow graphql@16 as peer dependency

[benjamn]

Similar to #60 and should fix apollographql/apollo-client#9199.

I decided against actually updating the graphql dev dependency to v16 because there are other dev dependencies (like apollo-cache-inmemory) that don't currently accept graphql@16 as a peer dependency, and I don't want to fall down the rabbit-hole of updating every old AC2-era package (unless that's our only option).

Fortunately, this package makes limited use of the graphql package, so it is indeed still compatible with v16:

apollo-link-persisted-queries/src/index.ts

Lines 3 to 9 in 88c3db2

	import { print } from 'graphql/language/printer';
	import {
	DefinitionNode,
	DocumentNode,
	ExecutionResult,
	GraphQLError,
	} from 'graphql';

I updated @types/graphql to its latest version to verify this usage of graphql is consistent with the latest types, at least.

[hwillson]

Great solution @benjamn - thanks very much!