Add SubscriptionNotification deprecation comments

[ercling]

Deprecation comments added for the following top-level objects: latest_receipt, latest_receipt_info, latest_expired_receipt, latest_expired_receipt_info. Documentation: https://developer.apple.com/documentation/appstoreservernotifications/

[codecov]

Codecov Report

Merging #111 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #111   +/-   ##
=======================================
  Coverage   90.36%   90.36%           
=======================================
  Files           4        4           
  Lines         249      249           
=======================================
  Hits          225      225           
  Misses         16       16           
  Partials        8        8

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[jun06t]

thanks @ercling :-)