chore: raise tsconfig target to ES2021

[bmish]

BREAKING CHANGE:
Updates the internal TSConfig target to ES2021.

Fixes #5980.

I didn't see any other production files to update this in. All the other tsconfig.json files present are test fixtures. Let me know if we should update those too.

Potential breaking change targeting v6 (#5886).

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[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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[JoshuaKGoldberg]

Nice, glad this was all that was needed. Thanks for sending this in!