feat!: upgrade Node.js to 14 LTS

[VerteDinde]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

This PR upgrades the Node version used in CI from 12 to 14.
If this upgrade goes smoothly, it may also be worth adding a CI set to test Node 16 (and eventually Node 18 when LTS starts in October).

[codecov]

Codecov Report

Merging #2921 (ac836a2) into master (18156d8) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2921      +/-   ##
==========================================
+ Coverage   71.61%   71.69%   +0.08%     
==========================================
  Files          79       79              
  Lines        2392     2392              
  Branches      450      450              
==========================================
+ Hits         1713     1715       +2     
  Misses        454      454              
+ Partials      225      223       -2     

Impacted Files	Coverage Δ
packages/maker/appx/src/MakerAppX.ts	60.71% <0.00%> (+1.78%)	⬆️
packages/api/cli/src/util/check-system.ts	70.73% <0.00%> (+2.43%)	⬆️

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[erickzhao]

CI passes => LGTM