Upgrade ESLint to version 9 (#5567)

This PR upgrades the ESLint version from 8 to 9 by first switching to
flat config format because version 9 made this format required.

There is a migration guide here:
https://eslint.org/docs/latest/use/configure/migration-guide

I also fixed some minor issues we have inside the eslint config while
there.

Author: canova