CTC amounts for 2021 in policy_current_law.json need correction

[towashington]

Note: This issue seems related to issue #2621, though not exactly the same.

Description: The ARPA lifted the CTC amounts for (and only for ) 2021. See details here, but:

1. For tax year 2021, the CTC increased from $2,000 per qualifying child to $3,600 for children ages 5 and under at the end of 2021, and $3,000 for children ages 6 through 17 at the end of 2021, but
2. The $500 nonrefundable Credit for Other Dependents amount has not changed.

Accordingly, the 2021 value of param CTC_c in policy_current_law.json should be corrected.

[martinholmer]

@towashington, Can you provide a simple test case for which Tax-Calculator gets the wrong amounts in 2021 for any of the following variables?

ctc_total
ctc_nonrefundable
ctc_refundable

[martinholmer]

@towashington, Are you aware of the fact that Tax-Calculator has several CTC_new_ policy parameters that came into play for 2021 only?