feat!: drop node8 support

[alexander-fenster]

BREAKING CHANGE: The library now supports Node.js v10+. The last version to support Node.js v8 is tagged legacy-8 on NPM.

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[codecov]

Codecov Report

Merging #562 into master will increase coverage by 12.62%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master     #562       +/-   ##
===========================================
+ Coverage   83.86%   96.48%   +12.62%     
===========================================
  Files          14        7        -7     
  Lines        3365     1281     -2084     
  Branches       87       95        +8     
===========================================
- Hits         2822     1236     -1586     
+ Misses        541       43      -498     
  Partials        2        2

Impacted Files	Coverage Δ
src/helpers.ts	97.67% <100%> (ø)	⬆️
src/v1/speech_client.ts	96.42% <100%> (+1.85%)	⬆️
src/v1p1beta1/speech_client.ts	96.44% <100%> (+2.05%)	⬆️
samples/quickstart.js
samples/transcribeContextClasses.js

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