ci: refactor workflows for release branching

[jescalada]

Closes #1517.

This refactors the release drafting and NPM publisher automation to work with release branches, as described in #1516. I've already updated #1516 with a step-by-step guide and a troubleshooting section based on my own testing.

Although it's a bit of a mess, I've replicated the release process in my git-proxy fork:

• Test releases made from drafts
• NPM test package built from those releases
• Related PRs showcasing the branching flow

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (4c6e8d4) to head (aa6db4f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1520   +/-   ##
=======================================
  Coverage   90.25%   90.25%           
=======================================
  Files          69       69           
  Lines        5561     5561           
  Branches      958      958           
=======================================
  Hits         5019     5019           
  Misses        523      523           
  Partials       19       19           

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[jescalada]

@kriswest Would be great to merge this in as we prepare for the 2.1 release! I've already made the release/2.0 branch from the 2.0.0 bump PR. Since these CI workflow updates haven't been merged yet, nothing has executed when making the release/2.0 branch (which is fine, since we decided to do everything manually one last time).