[eslint-config-terra] Update to ESLint v7.0.0

[mjhenkes]

Feature Request

Description

ESLint 7.0.0 was released. We should look at upgrading. We're currently blocked by projects like airbnb but won't be forever.
https://eslint.org/blog/2020/05/eslint-v7.0.0-released

Additional Context / Screenshots

Airbnb eslint config v7 issue.
airbnb/javascript#2223

@ Mentions

[ghost]

Airbnb has included ESLint 7 into their package which now causes eslint-config-terra to throw a dep warning.

https://github.com/airbnb/javascript/blob/master/packages/eslint-config-airbnb/package.json#L78