ReceiptValidator: rename `notAfter` to `notBefore`

[veehaitch]

So far, the functions validateReceiptAsync and validateReceipt had been accepting an argument notAfter which denoted a timestamp. If a receipt contains a timestamp which is before this deadline, it is considered expired. Therefore, the argument should have the name notBefore. This is also in line with X.509 certificates which use the same name to mark the beginning of the validity period.

Please note that the actual logic is unchanged and has been working correctly.

Fixes #37.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (cb26211) 97.16% compared to head (a9d384c) 97.16%.

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Additional details and impacted files

@@            Coverage Diff            @@
##               main      #38   +/-   ##
=========================================
  Coverage     97.16%   97.16%           
  Complexity      112      112           
=========================================
  Files            18       18           
  Lines           634      635    +1     
  Branches         96       96           
=========================================
+ Hits            616      617    +1     
  Partials         18       18           

Impacted Files	Coverage Δ
.../devicecheck/appattest/receipt/ReceiptValidator.kt	94.11% <100.00%> (+0.11%)	:arrow_up:

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