chore!: drops node 12 support

[sverweij]

Description

• See title
• 🚨 BREAKING CHANGE

Motivation and Context

• so we can use nullish coalescing etc and drop some dependencies
• upem is only used as a devDependency - and probably only by the tools listed on github's dependents tab, which use node 16 as a default environment.

How Has This Been Tested?

• automated non-regression tests
• green ci

Types of changes

• Chore (dependency & ci housekeeping for instance)
• Bug fix (non-breaking change which fixes an issue)
• Documentation only change
• Refactor (non-breaking change which fixes an issue without changing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• 📖

  • My change doesn't require a documentation update, or ...
  • it does and I have updated it

• ⚖️

  • The contribution will be subject to The MIT license, and I'm OK with that.
  • The contribution is my own original work.
  • I am ok with the stuff in CONTRIBUTING.md.