chore: upgrade preview Node.js from 12 to 14

[jawnsy]

Update the preview build to use Node.js 14.

[shortcut-integration]

This pull request has been linked to Shortcut Story #8108: Upgrade to Node.js 14 or latest LTS version.

[github-actions]

✨ Coder.com for PR #4255 deployed! It will be updated on every commit.

• Host: https://codercom-54p2d7f63-codercom.vercel.app/docs/code-server
• Last deploy status: success
• Commit: 52e2618
• Workflow status: https://github.com/cdr/code-server/actions/runs/1280565735

[codecov]

Codecov Report

Merging #4255 (52e2618) into main (edb3b41) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4255   +/-   ##
=======================================
  Coverage   65.09%   65.09%           
=======================================
  Files          36       36           
  Lines        1882     1882           
  Branches      380      380           
=======================================
  Hits         1225     1225           
  Misses        559      559           
  Partials       98       98           

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