feat: upgrade to ESLint v7

[bradzacher]

Lots of broken tests, a few copied enhancements, otherwise not that bad.
Didn't realise we hadn't updated the integration test image in a while.. it was still on node 8!

Fixes #1550

[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 #2022 into v3 will decrease coverage by 0.02%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##               v3    #2022      +/-   ##
==========================================
- Coverage   93.64%   93.61%   -0.03%     
==========================================
  Files         169      169              
  Lines        7677     7680       +3     
  Branches     2193     2195       +2     
==========================================
+ Hits         7189     7190       +1     
- Misses        229      231       +2     
  Partials      259      259              

Flag	Coverage Δ
#unittest	93.61% <33.33%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
...gin-internal/src/rules/no-poorly-typed-ts-props.ts	88.88% <ø> (ø)
...kages/eslint-plugin/src/rules/func-call-spacing.ts	96.87% <ø> (ø)
...kages/eslint-plugin/src/rules/no-empty-function.ts	88.46% <ø> (ø)
...nt-plugin/src/rules/space-before-function-paren.ts	100.00% <ø> (ø)
.../experimental-utils/src/eslint-utils/RuleTester.ts	9.75% <0.00%> (-0.51%)	⬇️
...ackages/eslint-plugin/src/rules/no-implied-eval.ts	97.87% <100.00%> (+0.04%)	⬆️