Drop Node v10/v13/v15 support

[mdjermanovic]

Since we are not planning to use Espree v8 in ESLint v7, can we align engines.node in package.json with ESLint v8 (eslint/eslint#14592)?

[aladdin-add]

and some other repos:

• eslint/eslintrc
• eslint/eslint-scope (I would suggest not to change it, as it was used by webpack, which supports Node v10)
• eslint/eslint-visitor-keys

[nzakas]

Makes sense.