Upgrade eslint to version 9

This is unblocked now that all dependencies have been updated and the
flat configuration format (compatible with ESLint 8 and 9) was
introduced first. The following deprecation warnings during `npm
install` are resolved by this upgrade:

```
npm warn deprecated @humanwhocodes/[email protected]: Use @eslint/config-array instead
npm warn deprecated @humanwhocodes/[email protected]: Use @eslint/object-schema instead
npm warn deprecated [email protected]: This version is no longer supported. Please see https://eslint.org/version-support for other options.
```

Note that according to https://eslint.org/version-support ESLint 8 is
officially EOL now, and ESLint 9 has been released for over seven
months and is the only officially supported version.

Fixes mozilla#17928.