[graphql] Pull together significant changes to communicate

Pull together any significant changes that came out of upgrading to the graphql gem. List of MRs in #210556 (closed) encapsulate changes that were compatible with the older gem. MR [graphql] Convert to using the new query interp... (!27536 - merged) holds the changes that were not compatible.

• communicate to graphql group/channel
• update dev docs where necessary

/cc @alexkalderimis @gitlab-org/graphql-experts