feat: drop support for node v8

[bradzacher]

BREAKING CHANGE:

Fixes #1420

[typescript-eslint]

Thanks for the PR, @bradzacher!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #1997 into v3 will decrease coverage by 0.03%.
The diff coverage is 87.09%.

@@            Coverage Diff             @@
##               v3    #1997      +/-   ##
==========================================
- Coverage   93.91%   93.87%   -0.04%     
==========================================
  Files         171      172       +1     
  Lines        7787     7802      +15     
  Branches     2238     2237       -1     
==========================================
+ Hits         7313     7324      +11     
- Misses        217      221       +4     
  Partials      257      257              

Flag	Coverage Δ
#unittest	93.87% <87.09%> (-0.04%)	⬇️

Impacted Files	Coverage Δ
...xperimental-utils/src/eslint-utils/applyDefault.ts	100.00% <ø> (ø)
.../experimental-utils/src/ts-eslint-scope/analyze.ts	100.00% <ø> (ø)
packages/eslint-plugin/src/util/objectIterators.ts	66.66% <66.66%> (ø)
packages/eslint-plugin/src/rules/ban-types.ts	100.00% <100.00%> (ø)
...ackages/eslint-plugin/src/rules/member-ordering.ts	98.46% <100.00%> (ø)
...slint-plugin/src/rules/no-unnecessary-condition.ts	99.15% <100.00%> (ø)