Enable eslint flat config

[Jessy-BAER]

Eslint provides a new config file format with a better granularity. It could be great to use this new feature

https://eslint.org/docs/latest/use/configure/configuration-files-new

[yoannfleurydev]

I checked the documentation, as it is not enabled by default by ESLint, I prefer to stay on the conventional and old config at the moment.

I'll discuss this ticket with some other contributors to have their opinion on that.