Update CI to Xcode 26.2 - #6088

Fixes flutter/flutter#194285 (hopefully)

The main cause of flutter/flutter#194285 (where `Product.products(for:)` returned `[]` across StoreKit 2 tests on the iOS 27 simulator) was fixed in #13213 by preventing the example app from calling StoreKit APIs before `SKTestSession` initialized. However, `testGetProductsWithStoreKitError` and `testFailedNetworkErrorPurchase` remains fragile in iOS 26.2+

This is still because product caching in `StoreKitTest` is still 1. Opaque 2. unable to be manually cleared - the product cache sticks around even after calling `resetToDefaultState()`. Thus, the state of the cache depends on the order that the tests will run. :/ So, if a test that purchases a product succeeds, if we later hit `setSimulatedError(..., forAPI: .loadProducts)`, the cached product will be returned, causing the test to fail. 

This is an issue that RevenueCat also ran into as shown here: RevenueCat/purchases-ios#6088, where they skip tests using `.loadProduct` for anything above ios 26. Instead of keeping dead tests around, I think its fair to just remove them and avoid just keeping dead code around till the end of time. Additionally, the code paths tested by the removed tests are already being hit by `testFailedProductUnavilablePurchase`, so we arent really losing coverage. 

Additionally in this PR I fix up some leaky states between tests.

## Pre-Review Checklist