Return 406 on unsupported Accept header

Note: as it relates to RFC 7231, we are in compliance now -- see where it says "or disregard the header field...". Again, I like better the use of status code 406, but probably implement it in version 8, reconfiguring the middleware to perform content type negotiation prior to execution of the GraphQL request.

_Originally posted by @Shane32 in https://github.com/graphql-dotnet/server/pull/919#discussion_r974561756_