How do I tell Turbotax that I want the feds to keep some of my current overpayment to cover next years' estimated taxes?

First of all, those estimated tax vouchers that printed are just an estimate (suggestion) that TurboTax prepares based on your 2020 tax return.  These are not precise calculations, but rather suggested payments.

If you want to more precisely calculate what your quarterly estimated tax payments for 2021 should be, I'd suggest using the IRS publication I'm linking below.  You'll find in that publication worksheets that will be much more specific.

2021 Form 1040-ES (irs.gov)

That said, you certainly can apply any or all of your 2020 tax refund to apply toward your 2021 taxes.  See the instructions I'm linking below to get that done.

How do I apply my refund to next year's taxes? 

Please post back if you have further questions.