Update Google Play Billing Library to 9.1.0

[ianrumac]

Bumps the Google Play Billing Library from 8.0.0 to 9.1.0 and the RevenueCat SDK used by the sample apps from 9.2.0 to 10.14.1.

Billing 9 removes the SkuDetails and purchase-history APIs entirely (see the Play Billing Library 9 migration guide), so this PR also removes the SDK surface that depended on them.

Changes in this pull request

• billing_version 8.0.0 → 9.1.0 in gradle/libs.versions.toml.
• revenue_cat_version 9.2.0 → 10.14.1 — used by the sample apps; required for Billing 9 compatibility.
• Removes SuperwallBillingFlowParams.Builder.setSkuDetails(SkuDetails) (was deprecated) — SkuDetails no longer exists in Billing 9. Callers should use setProductDetailsParamsList(...) with ProductDetails.
• Removes the internal SWProduct wrapper — unused, and it wrapped the removed SkuDetails type.
• buildQueryPurchaseHistoryParams now builds QueryPurchasesParams instead of the removed QueryPurchaseHistoryParams, resolving current purchases rather than purchase history.
• Raises minSdk to 23 across all modules (SDK was 21, sample/compose modules 22/23) — Billing 9 requires Android 6.0 (API 23).
• Bumps SUPERWALL_VERSION to 2.8.0 and adds a CHANGELOG entry with upgrade guidance.

Impact on SDK users

• minSdk is now 23. Apps with a lower minSdk must raise it to pick up this release; Android 5.x devices no longer receive updates that include this SDK version.
• Because Gradle resolves the Billing Library to a single version across the app, upgrading Superwall upgrades Billing for everything else that depends on it. Apps that still call removed Billing APIs (SkuDetails, SkuDetailsParams, querySkuDetailsAsync, queryPurchaseHistoryAsync, BillingClient.SkuType, no-arg enablePendingPurchases()) will stop compiling, and other billing SDKs in the app (RevenueCat, Adapty, Qonversion, …) must support Billing 9. The CHANGELOG entry calls this out and asks users to test purchase/restore/subscription-status flows end to end before shipping the upgrade.

Checklist

• All unit tests pass.
• All UI tests pass.
• Demo project builds and runs.
• I added/updated tests or detailed why my change isn't tested.
• I added an entry to the CHANGELOG.md for any breaking changes, enhancements, or bug fixes.
• I have run ktlint in the main directory and fixed any issues.
• I have updated the SDK documentation as well as the online docs.
• I have reviewed the contributing guide