chore(ads): remove experimental flag from ads apis (#4068)

### Description

This change removes all the experimental annotations and comments around
ad tracking, admob adapter and ad reward features.

Many files - but pretty much the same change everywhere.


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Public API surface change: ads APIs no longer require opt-in. Behavior
is unchanged, but this is a compatibility/contract graduation that
callers will rely on.
> 
> **Overview**
> Graduates ad tracking, AdMob adapter helpers, and reward-verification
APIs out of experimental status. Callers no longer need
`@OptIn(ExperimentalPreviewRevenueCatPurchasesAPI)`.
> 
> `Purchases.adTracker`, `AdTracker` event types,
`pollRewardVerification` / `awaitPollRewardVerification`,
`generateRewardVerificationToken`, and AdMob `loadAndTrack*` /
reward-verification extensions are now unmarked stable APIs. Sample apps
and docs drop the experimental notices and opt-in compiler flags. No
runtime behavior changes.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
da8456500f7b96bc6a335c2eda1deff279736195. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->