Upgrade eslint to latest version and fix all linting problems

Current eslint version [has a bug](https://github.com/eslint/eslint/issues/12117) that will give false positives about variables defined inside `for` loops not being used. Upgrading to the latest eslint version ( and, apparently, removing the eslint cache too) should get rid of this problem.

Also, all current eslint errors should be addressed too.

Discussed here: https://github.com/Automattic/woocommerce-services/pull/2094

Start by reviewing the project's ESLint version and cache configuration, then inspect the current lint output. The discussion in pull request 2094 and the linked ESLint bug provide context for the upgrade. Done means upgrading ESLint, clearing the cache if needed, and addressing all current linting errors.

Written by the indexing model from the issue text.