chore: drop node 6 and 8 support

[erisu]

Motivation and Context

• Follow inline with Node Support
• Prepare for next major
• Add Node 12 & Drop Node 6 & 8 Support cordova#79

Description

• Removes Node 6 and 8 from CI Testing Services
• Bump engines.node to >=10 in package.json

Testing

• npm t
• CI services continue to pass

Checklist

• I've run the tests to see all new and existing tests pass

[codecov-io]

Codecov Report

Merging #832 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #832   +/-   ##
=======================================
  Coverage   66.29%   66.29%           
=======================================
  Files          19       19           
  Lines        1860     1860           
=======================================
  Hits         1233     1233           
  Misses        627      627

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[timbru31]

LGTM, but this please do not merge yet, as this is a BREAKING CHANGE

[erisu]

Since the repo has been bumped (#890) for next major, I will continue with merging in this PR.