Scripts: Upgrade ESLint to the new major version (7.x)

[gziolo]

Description

ESLint v7.0.0 was released:

https://eslint.org/blog/2020/05/eslint-v7.0.0-released

We should update our packages to use the latest version. It has a few breaking changes as highlighted here:
https://eslint.org/blog/2020/05/eslint-v7.0.0-released#breaking-changes

One of the issues resolved (eslint/eslint#11558) allows us to remove the following workaround added in #17744:

gutenberg/packages/scripts/config/.eslintrc.js

Lines 6 to 10 in 605cb2b

	// Temporary workaround to use linting rules for both e2e and unit tests with all files
	// until override files globbing logic is fixed when using with --config.
	// @see https://github.com/eslint/eslint/issues/11558
	'plugin:@wordpress/eslint-plugin/test-e2e',
	'plugin:@wordpress/eslint-plugin/test-unit',

References

It might be helpful to look at previous PR where minor version upgrade for ESLint was applied: #21424. The changes included:

• upgrade of ESLint package to the latest version
• bringing all ESLint plugins and configs used to the latest version
• listing all changes applied in the relevant changelog files
• fixing any new errors that might pop up

[aduth]

Somewhat related: #16795 (comment)

[ocean90]

@gziolo In #22771 I have started with the upgrade. We may have to wait for two dependencies getting their update to avoid the warning.

There are some new JS lint errors which seem to be valid. Can you confirm that they are correct and should be fixed? I suppose a separate PR is preferred?

[gziolo]

There are some new JS lint errors which seem to be valid. Can you confirm that they are correct and should be fixed? I suppose a separate PR is preferred?

@ocean90, thank you for starting this PR. I left my detailed comment there. I think those are valid issues that need to be addressed and we definitely need to split into more PRs. How we proceed, it's up for debate :)