Fix wage growth rates

[martinholmer]

This pull request completes the revisions that began with #1164 and continued in #1167. The hard-coded Policy.__wgrates have been changed so that they agree with the rate of growth in per-capita wages-and-salaries implied by the projection factors in the StageIfactors.csv file. The only use of these rates is to adjust the social security maximum taxable earnings policy parameter, _SS_Earnings_c. So, the results changes caused by this reduction in Policy.__wgrates values are limited to social security results. For example, in 2022 total payroll taxes are reduced by $8.8 billion, which is a decline of about two-thirds of one percent. Notice there are changes only in 2018 and subsequent years because actual (not projected) values for _SS_Earnings_c are used in years through 2017.

@MattHJensen @andersonfrailey @codykallen @Amy-Xu

[codecov-io]

Codecov Report

Merging #1171 into master will not impact coverage.

@@           Coverage Diff           @@
##           master    #1171   +/-   ##
=======================================
  Coverage   98.88%   98.88%           
=======================================
  Files          38       38           
  Lines        3062     3062           
=======================================
  Hits         3028     3028           
  Misses         34       34

Impacted Files	Coverage Δ
taxcalc/policy.py	100% <100%> (ø)	✅

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