feat: add support for GitHub Enterprise Server (GHES) v3.6 (#469)

This adds support for GitHub Enterprise Server v3.6.

This changes were generated locally with the changes from a
[recent PR][1] to `gr2m/github-openapi-graphql-query` applied to
add compatibility with v3.6.

[1]: gr2m/github-openapi-graphql-query#68

Author: timrogers