Suggesting to use node 12 as default

[sairus2k]

Summary

Now in CONTRIBUTING.md, it is recommended to use node 10.x for development because of this bug #8490. Since that bug was fixed and node 12 became LTS, I suggest switching to node 12 as a recommendation for contributing and linting.

What this PR does:

1. Fix lint errors in node 12
2. Update CircleCI config to use node 12 for linting
3. Update contributing documentation

close #9015

Test plan

Green CI

[thymikee]

That's aligned with #9088. Let's wait till it's merged :) cc @SimenB

[SimenB]

I'm down with this as we really should be green for people on the latest lts 👍

[codecov-io]

Codecov Report

Merging #9098 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #9098   +/-   ##
=======================================
  Coverage   64.69%   64.69%           
=======================================
  Files         277      277           
  Lines       11701    11701           
  Branches     2874     2873    -1     
=======================================
  Hits         7570     7570           
  Misses       3514     3514           
  Partials      617      617

Impacted Files	Coverage Δ
...kages/jest-worker/src/workers/NodeThreadsWorker.ts	90.27% <ø> (ø)	⬆️
packages/jest-worker/src/workers/threadChild.ts	84.31% <ø> (ø)	⬆️

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[github-actions]

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