feat(eslint-plugin): [no-floating-promises] ignore void operator by default

[bradzacher]

eslint/eslint#12613 was released in eslint v7, so it's now possible and easy to use this within the ecosystem.
I think this makes sense as a way to consume top-level promises, so changing it to true.

[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.

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[codecov]

Codecov Report

Merging #2003 into v3 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##               v3    #2003   +/-   ##
=======================================
  Coverage   93.75%   93.75%           
=======================================
  Files         176      176           
  Lines        7814     7814           
  Branches     2240     2240           
=======================================
  Hits         7326     7326           
  Misses        228      228           
  Partials      260      260           

Flag	Coverage Δ
#unittest	93.75% <ø> (ø)

Impacted Files	Coverage Δ
...es/eslint-plugin/src/rules/no-floating-promises.ts	100.00% <ø> (ø)