Drop Node.js 6 support

[b-admike]

Description

LoopBack4 is dropping support for Node 6 because it is no longer LTS and doesn't support newer features. It's best for extensions to align with it.

BREAKING CHANGE: Support for Node.js version lower than 8.0 has been dropped. Please upgrade to Node 8 or higher.

Related issues

• follow up to https://github.com/strongloop/loopback4-extension-grpc/commit/65414e1dc9e55dbdcee309e7f1ece7f9fde65792
• related to https://github.com/strongloop/loopback-next/issues/611

Checklist

• [x] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide

[b-admike]

@jonathan-casarrubias @VMois PTAL

[VMois]

It looks good. I think we don't need to do something about CLI, or delete dist6 from prettier or gitignore. I will merge this pull request.