Upgrade to webonyx/graphql-php v15.x

[Kingdutch]

As of January 6th, graphql-php 15.0.0 has been released. The change list is quite long but not overly intimidating. Looking at the changes that I think affect the GraphQL module we should be able to update in a reasonable time-frame with not too much effort. However, I expect that some of our more advanced users will be affected by the breaking changes so I do believe this update requires tagging a major version.

The changes in 15.0.1 -> 15.2.5 are all minor so we can easily jump straight to the latest version rather than starting at 15.0.0

[mxr576]

Is anybody working on this?