Upgrades Eslint to v9 (#6694)

## What's the problem this PR addresses?

We're still relying on ESLint 8, which isn't supported anymore.

Fixes #6687, #6689

Supercedes #6693

## How did you fix it?

This PR upgrades ESLint to its latest version, switching to the
stylistic rules when needed. No rules were removed.

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Author: arcanis