[readme] remove global usage and eslint version from readme

[aladdin-add]

I've also updated eslint@7 => eslint@8(latest).

[codecov-commenter]

Codecov Report

Merging #3254 (5bc571d) into master (2145e29) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3254   +/-   ##
=======================================
  Coverage   97.67%   97.67%           
=======================================
  Files         121      121           
  Lines        8575     8575           
  Branches     3112     3112           
=======================================
  Hits         8376     8376           
  Misses        199      199           

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[ljharb]

Thanks! I just removed the version; the default is "latest" and this way we won't have to keep updating it.