electron 21

[jackkav]

• fixes nan issue [Bug]: Native modules that include nan.h fail to build on electron 20 electron/electron#35193
• fixes github action issue [Ubuntu] homebrew will be removed from PATH variable actions/runner-images#6283
• bumps node-gyp, electron and package.json version
• removes support for node versions 12 and 14
• forces openssl version to 1.1.1r to avoid issue 1.1.1q test/v3ext.c fails to compile on MacOS (missing string.h include) openssl/openssl#18720
• removes some ::set-output which are deprecated by Github
• updates used GH actions runners to use windows-latest, ubuntu-latest and macos-latest
• removes appveyor, circleci and travis pipeline code

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (develop@e526ca0). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop       #6   +/-   ##
==========================================
  Coverage           ?   93.51%           
==========================================
  Files              ?       44           
  Lines              ?      940           
  Branches           ?       97           
==========================================
  Hits               ?      879           
  Misses             ?       41           
  Partials           ?       20           

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