message: update changelog for release 2.27.0
diff: - <First `apollo-codegen-swift` related entry goes here>
  - `apollo-codegen-typescript`
  - <First `apollo-codegen-typescript` related entry goes here>
  +- `apollo-codegen-core`
  + - <First `apollo-codegen-core` related entry goes here>
  - `apollo-env`
  - <First `apollo-env` related entry goes here>
  - `apollo-graphql`
  - `vscode-apollo`
  - <First `vscode-apollo` related entry goes here>
  +## `apollo@2.27.0`
  +
  +- `apollo@2.27.0`
  + - downgrade mkdirp to keep node 8 working as expected [63f0773](https://github.com/apollographql/apollo-tooling/commit/63f077313533c7aa304464bf9c02bb7c3dba4edc)
  + - Remove "hidden: true" from --variant in service:push [#1865](https://github.com/apollographql/apollo-tooling/pull/1846)
  + - update ENGINE_API_KEY deprecation message to give alternative [#1866](https://github.com/apollographql/apollo-tooling/pull/1866)
  + - Add json-modern codegen target which adds typeNodes (ast type nodes) to codegen output [#1846](https://github.com/apollographql/apollo-tooling/pull/1846)
  +- `apollo-codegen-core@0.36.6`
  + - Add json-modern target which adds typeNodes (ast type nodes) to codegen output [#1846](https://github.com/apollographql/apollo-tooling/pull/1846)
  +
  ## `apollo@2.26.0`
  - `apollo@2.26.0`
  