chore: drop support for ESLint v6

[bmish]

BREAKING CHANGE:
Drops support for ESLint v6.

Fixes #5968.

Breaking change targeting v6 (#5886).

Also changed the ESLint v6 integration test to test ESLint v7 instead.

[typescript-eslint]

Thanks for the PR, @bmish!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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✅ Successfully ran 46 targets

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[bmish]

Seems like we might as well include the constraint here too.

[JoshuaKGoldberg]

Looks great, thanks!