Update JavaScript / ESLint guidance

[coliff]

ESlint released a major new version (v9) with many big changes... including a new config file format (the old .eslintrc config is now deprecated).

I think we should make a PR to the Devguide update our guidelines to match this since the new flat config is also compatible with ESLint v8.

I also recommended changing our base config to use eslint-recommended and not Google's which is now deprecated and hasn't been updated in 5 years (I just discovered this today!)