docs: update supported TS version in readme

[dnalborczyk]

#2057

[typescript-eslint]

Thanks for the PR, @dnalborczyk!

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 #2059 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2059   +/-   ##
=======================================
  Coverage   93.40%   93.40%           
=======================================
  Files         171      171           
  Lines        7745     7745           
  Branches     2210     2210           
=======================================
  Hits         7234     7234           
  Misses        246      246           
  Partials      265      265           

Flag	Coverage Δ
#unittest	93.40% <ø> (ø)

[bradzacher]

Thanks!