feature: Upgrade Flutter SDK to the upcoming release 3.39.0 to support Visual Studio 2026 builds

[Creeper19472]

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Describe the requested feature

Visual Studio 2026 has been officially released recently, however support for VS 2026 was only recently completed. This means that in order to support building Windows applications using VS 2026, we should prepare to upgrade the Flutter SDK version to its next stable version (3.39.0) as soon as possible.

Suggest a solution

Bump MINIMAL_FLUTTER_VERSION in flet-cli commands\build.py to the version of the next release.

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[FeodorFitsner]

We will update to 3.39 as soon as it's out. But it's going to be done in Flet v1 (0.70.x) only. Old 0.28.3 cannot be updated as Flutter API has changed a lot since 3.29. So, plan migrating to Flet v1.