fix(next): add defaults for iap local testing

[StaberindeZA]

Because

• IAP local testing showed no results in payments-next sub manage

This pull request

• Add defaults to payments-next .env for iap local testing
• Update create-mock-iap-subscriptions script to create app store with valid expiry date number, instead of string.

Issue that this pull request solves

Closes: (issue number)

Checklist

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• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

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