Support eslint 9+ in CI

[toofar]

Seems that a new major version of eslint has landed in the docker container we use for CI. eslint is just used to lint some small JS files, so it isn't a critical component, but it's making all CI runs report as failed atm, so I'll pin it to <9.0.0

This issue is for unpinning eslint again which looks like it'll require migrating our config file to the new format (and not much else), I've made a start on that here: https://github.com/qutebrowser/qutebrowser/tree/feat/eslint9

The migrating guide is here: https://eslint.org/docs/latest/use/migrate-to-9.0.0

Here's an example of a failing CI run: https://github.com/qutebrowser/qutebrowser/actions/runs/8576697138/job/23508116987