[in_app_purchase] Update Play Billing library to 7.1.1

[mchudy]

Updates Play Billing Library to the latest version 7.1.1. Exposes new APIs as per release notes:

• Adds Dart representation of ProductDetails.InstallmentPlanDetails
• Adds Dart representation of PendingPurchasesParams and removes the deprecated enablePendingPurchases method on BillingClientWrapper (breaking change)
• Adds Dart representation of Purchase.PendingPurchaseUpdate
• Removes the deprecated ProrationMode as it has been removed from the native library (breaking change)

This PR introduces breaking changes in in_app_purchase_android, but does not introduce any breaking changes on the platform interface level.

Fixes flutter/flutter#147394

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[stuartmorgan]

@reidbaker Ping on this review

[reidbaker]

@gmackall I am not going to get to this. Deferring to you for review.