## RevenueCat SDK
This release updates the SDK to use Google Play Billing Library 8. This
version of the Billing Library removed APIs to query for expired
subscriptions and consumed one-time products, aside from other
improvements. You can check the full list of changes here:
https://developer.android.com/google/play/billing/release-notes#8-0-0

Additionally, we've also updated Kotlin to 2.0.21 and our new minimum
version is Kotlin 1.8.0+. If you were using an older version of Kotlin,
you will need to update it.

Regarding API changes, we've also removed data classes from our public
APIs. This means that for classes that were previously data classes, the
`copy` function and `componentN` functions (destructuring declarations)
have been removed. `equals` and `hashCode` functions still work as
before.

### Play Billing Library 8: No expired subscriptions or consumed
one-time products

**Note:** the following is only relevant if you recently integrated
RevenueCat, and do not (yet) have all your transactions imported.
  
Play Billing Library 8 removed functionality to query expired
subscriptions or consumed one-time products. This means that, for users
migrating from a non-RevenueCat implementation of the Play Billing
Library, the SDK will not be able to send purchase information from
these purchases. We can still ingest historical data from these
purchases through a backend historical import. See
[docs](https://www.revenuecat.com/docs/migrating-to-revenuecat/migrating-existing-subscriptions).
This doesn't affect developers that have all transactions in RevenueCat,
which is true for the vast majority.

### Bumped minimum Kotlin version

RevenueCat SDK v9 bumps Kotlin to 2.0.21, with a minimum Kotlin version
of 1.8.0.

### Using the SDK with your own IAP code (previously Observer Mode)

Using the SDK with your own IAP code is still supported in v9. Other
than updating the SDK version, there are no changes required. Just make
sure the version of the Play Billing Library is also version 8.0.0+.

### 💥 Breaking Changes
* Removes data classes from public API (#2498) via JayShortway
(@JayShortway)
* Marks `PaywallData` and `PaywallColor` as `InternalRevenueCatAPI`.
(#2507) via JayShortway (@JayShortway)
* BC8 migration (#2506) via Toni Rico (@tonidero)
* Update to kotlin 2.0.21 while keeping language compatibility (#2493)
via Toni Rico (@tonidero)

### 🔄 Other Changes
* Update CustomEntitlementComputation sample app kotlin version (#2510)
via Toni Rico (@tonidero)
* Fix `Switch` component previews (#2509) via Toni Rico (@tonidero)
* Add V9 migration guide (#2508) via Toni Rico (@tonidero)
* [AUTOMATIC][Paywalls V2] Updates paywall-preview-resources submodule
(#2499) via RevenueCat Git Bot (@RCGitBot)

---------

Co-authored-by: JayShortway <29483617+JayShortway@users.noreply.github.com>

### Description
Upgrades/downgrades were not working after the changes in #2506. This
fixes the issue and adds some tests to the relevant code to avoid it
from happening again.

This should deal with #2531