Target Node.js v14 instead of v13

[mangs]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

This is my first contribution. I'm adding GitHub actions at my job, and I figured I would investigate how Rollup does it; in turn, I noticed this potential oversight.

Because Node.js v13 has been superseded by v14 with the intention to make the latter an LTS version, builds should target the eventual LTS version.

[codecov]

Codecov Report

Merging #3585 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3585   +/-   ##
=======================================
  Coverage   96.30%   96.30%           
=======================================
  Files         177      177           
  Lines        6065     6065           
  Branches     1786     1786           
=======================================
  Hits         5841     5841           
  Misses        112      112           
  Partials      112      112           

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