🛠 Repo: Bump ESLint config languageOptions.ecmaVersion to >=2020

[JoshuaKGoldberg]

Tooling Suggestion Checklist

• I have tried restarting my IDE and the issue persists.
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Overview

mocha/eslint.config.js

Line 15 in 51502ab

ecmaVersion: 2018,

Mocha supports Node 14. Which, per https://github.com/tsconfig/bases/blob/8fee7129ada13c7c779472ebb29130fa9581cf1c/bases/node14.json, means we can support ES2020 in internal linting. Yay!

Additional Info

Context: #4829 (comment).