fix: replace scrollableBuilder with builder in CupertinoSheetRoute fo…

[Pranav2918]

The CupertinoSheetRoute now correctly implements the required builder signature according to the Flutter API.

// Before
CupertinoSheetRoute<void>(
  scrollableBuilder: (BuildContext context, ScrollController controller) {
    // ...
  },
)

// After
CupertinoSheetRoute<void>(
  builder: (BuildContext context) {
    // ...
  },
)

Changes made:

• Updated the CupertinoSheetRoute constructor to use the correct builder parameter instead of the undefined scrollableBuilder parameter.

• Refactored the builder function to remove the unused ScrollController and simplified the internal structure for better readability.

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• [✓] I read the Flutter Style Guide recently, and have followed its advice.
• [✓] I signed the CLA.
• [✓] I read the Contributors Guide.
• [✓] I have added sample code updates to the changelog.
• [✓] I updated/added relevant documentation (doc comments with ///).

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