[various] Update all packages to Pigeon 26 (#10450)
Pigeon versions <26.0.4 aren't compatible with analyzer 8.x, which is causing conflicts for flutter/flutter, so this updates the entire repo to 26.
Related: https://github.com/flutter/flutter/issues/178270
Manual fixes:
- Updates camera_android_camerax and webview_flutter_android for https://github.com/flutter/packages/pull/9515
- Adds the pigeonVar_* overrides to the url_launcher_windows tests
- Removes equality extensions to Pigeon data classes in in_app_purchase_storekit, as they are now generated by Pigeon, causing redeclaration errors.
## Pre-Review Checklist
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[interactive_media_ads] Adds support for accessing data for an Ad (#9972)
## Pre-Review Checklist
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