Always use `context.graphql.schema` for `createExpressServer`

This pull request fixes createExpressServer to always use context.graphql.schema instead of the GraphQLSchema argument passed in.

Users downstream will nearly always be interacting with their GraphQL schema using the context.query and context.db methods. As part of https://github.com/keystonejs/keystone/pull/9028 we have deprecated the GraphQLSchema argument that was provided, but if users are using that instead of context.graphql.schema, then I think it is a safe assumption that downstream users would have expected that the the GraphQL schema provided should have been equal to the operating schema of context.

This could arguably be considered a breaking change, but I don't think anyone would have reasonably used the argument of createExpressServer (deprecated) to differentiate between the two possible schemas, therefore I think this is a patch that will help developers if they made an error in passing a different GraphQLSchema through.

keystonejs / keystone

Always use `context.graphql.schema` for `createExpressServer` #9029