DEP Allow graphql ^6

[emteknetnz]

Issue silverstripe/.github#191

[GuySartorelli]

We probably need to decide how we want to handle this one - maintaining it will be easier if we don't do multi-branch support like this.

[emteknetnz]

Why would it be easier? This PR seems easier.

[GuySartorelli]

Any breaking changes in other graphql 6 may cause problems with this. It's the same reason we bump other modules like config which may not directly need it.

[emteknetnz]

I'd say that's graphql 6 is very unlikely to break this. There hasn't been any discussion for work on GraphQL 6 so most likely scenarios is that it will be near identical to GraphQL 5

If we do run into problems at a future then we can access whether we should adapt the 1 branch of this module to add support for GraphQL 6, or if we should create a new 2 branch instead

[GuySartorelli]

Seems weird that we're handling this one differently than everything else, when the argument was that we should just handle everything the same way... but w/e.