Update dependency flutter to v3.38.6

[renovate]

This PR contains the following updates:

Package	Update	Change
flutter (source)	patch	3.38.5 → 3.38.6

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Release Notes

flutter/flutter (flutter)

v3.38.6

Compare Source

• flutter/179139 - flutter widget-preview start creates new cached build artifacts on each run, resulting in increasing disk usage after each run.
• flutter/178896 - Apps crash during launch on Windows when run from paths containing non-ASCII characters.
• flutter/176943 - Configuration changes to run tests on macOS 15 or 15.7.2 for Flutter's CI.
• flutter/179914 - Flutter Android apps that upgrade to AGP 9.0.0 require migration steps.
• flutter/175099 - When WebViews are scrolled on iOS 26, they become unclickable.
• flutter/175074 - When the virtual keyboard is closed on Android web, the area behind it remains blank and the app only draws in the area that used to be above the keyboard.
• flutter/180381 - Apps crash on Android when enabling accessibility, hiding a platform view, and pulling out the top curtain.

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