🚀 Feature: Drop support for Node 16

[JoshuaKGoldberg]

Bug Report Checklist

• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

pnpm 9.x does not supports Node 16 per https://pnpm.io/9.x/installation: it's now v18.12 and higher. Which means this repo's maintenance must choose between using an older pnpm or dropping v16 support.

Per https://endoflife.date/nodejs, Node 16 went EOL over a year ago. I choose dropping Node 16 support in a 2.x of ts-api-utils.

Additional Info

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[JoshuaKGoldberg]

👋 @RebeccaStevens are there any breaking changes you'd want to make while we're in the area?