SOLIDUI: Migrate to using file_picker 11.0.2 upgraded from 10.3.10

[gjwgit]

There is no platform. Probably replace FilePicker.platform.saveFile() with FilePicker.saveFile()

Confirm and test then PR

  error • The getter 'platform' isn't defined for the type 'FilePicker' • lib/src/utils/solid_file_operations_download.dart:144:45 • undefined_getter
  error • The getter 'platform' isn't defined for the type 'FilePicker' • lib/src/utils/solid_file_operations_download.dart:313:43 • undefined_getter
  error • The getter 'platform' isn't defined for the type 'FilePicker' • lib/src/utils/solid_file_operations_upload.dart:59:39 • undefined_getter
  error • The getter 'platform' isn't defined for the type 'FilePicker' • lib/src/widgets/solid_file_operations.dart:143:45 • undefined_getter
  error • The getter 'platform' isn't defined for the type 'FilePicker' • lib/src/widgets/solid_file_uploader.dart:109:37 • undefined_getter
  error • The getter 'platform' isn't defined for the type 'FilePicker' • lib/src/widgets/solid_file_uploader_helpers.dart:46:49 • undefined_getter