Support BC7

[tonidero]

Description

This PR adds support for BC7 and related new features.

New APIs:

• SubscriptionOption.installmentInfo which contains information about installment plans you might want to display in your paywall.
• PurchasesConfiguration.pendingTransactionsForPrepaidPlansEnabled allows to enable pending prepaid subscriptions.

Breaking changes

• Increase min Sdk to 21. This was increased in BC7
• Remove UpgradeInfo
• Remove all purchasePackage and purchaseProduct methods using UpgradeInfo. If you want to perform upgrades/downgrades, please migrate to use the purchase(PurchaseParams) method, which has options to perform that operation.

[RevenueCat-Danger-Bot]

	1 Message
:book:	Size increase: 13.74 KB

Generated by :no_entry_sign: Danger

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.78378% with 6 lines in your changes missing coverage. Please review.

Please upload report for BASE (8.0.0-dev@4a7582b). Learn more about missing BASE report.

Files	Patch %	Lines
.../kotlin/com/revenuecat/purchases/common/Backend.kt	50.00%	3 Missing and 2 partials :warning:
...om/revenuecat/purchases/models/TestStoreProduct.kt	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 8.0.0-dev #1737 +/- ## ============================================ Coverage ? 82.99% ============================================ Files ? 222 Lines ? 7576 Branches ? 1070 ============================================ Hits ? 6288 Misses ? 872 Partials ? 416 ```

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