[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.
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[camera] Remove deprecated feature from Windows example (#10380)
The Windows camera example was showing usage of maxVideoDuration, but the parameter is deprecated and ignored, so shouldn't be part of the example.
Also makes a minor improvement to type safety of some code interacting with Pigeon, since the Pigeon definition had already been changed to non-nullable.
Fixes https://github.com/flutter/flutter/issues/166605
## Pre-Review Checklist
[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.