drop support for node 6

[nabdelgadir]

Description

Drop support for Node.js 6 and see if CI is green for Node 12

Related issues

• connect to https://github.com/strongloop/loopback-next/issues/3110

Checklist

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

[nabdelgadir]

db2 doesn't support Node 12 yet