build!: upgrade engines field to >=8.10.0

[bcoe]

This drops support for all Node.js versions < 8.10.0, please upgrade

BREAKING CHANGE: library now requires Node >= 8.10.0

[codecov]

Codecov Report

Merging #461 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #461   +/-   ##
=======================================
  Coverage   93.76%   93.76%           
=======================================
  Files          10       10           
  Lines         449      449           
  Branches       74       74           
=======================================
  Hits          421      421           
  Partials       28       28

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[evelant]

This breaks usage with firebase cloud functions. Firebase cloud functions require engines to be "8" or "10" and won't accept any other values. Due to that, this library fails to compile when attempting to deploy cloud functions firebase deploy --only functions with

i  functions: preparing functions directory for uploading...

Error: Error occurred while parsing your function triggers.

Error: @grpc/grpc-js only works on Node ^8.13.0 || >=10.10.0