docs: prepare documentation, changelogs, and security policy for v13.0.0

[vicajilau]

Description

Prepares documentation, changelogs, and security policies ahead of the 13.0.0 release:

• Root README.md:

  • Bumped Quick Start dependency from ^12.0.0 to ^13.0.0.
  • Noted that PlatformFile.length() returns Future<int?> and highlighted file.lengthSync().
  • Added a dedicated ## Migration section referencing both v13 and v12 migration guides.
  • Linked to migration guides under ## Documentation.

• SECURITY.md:

  • Updated the supported versions matrix to the new major versions (file_picker: Latest 13.x, file_picker_platform_interface: Latest 4.x, platform packages: Latest 2.x / Latest 4.x).

• packages/file_picker/CHANGELOG.md:

  • Documented the raised lower bounds on all platform implementation packages.
  • Added section highlighting the FilePickerWebOptions.readSequential fix (file_picker_web 4.0.0, file_picker_web: FilePickerWebOptions.readSequential has no effect #2206).
  • Added section documenting the removal of legacy native withData paths (android_file_picker 2.0.0, file_picker_darwin 2.0.0, Scope the next major version (13.0.0) #2202).

• packages/file_picker/README.md:

  • Updated pub package badge link to pub.dev.
  • Added tip regarding file.lengthSync() in the Migrating to v13 section.
  • Refined deprecated parameters wording to specify that allowMultiple, withData, withReadStream, and readSequential were removed from pickFiles().

• packages/file_picker_windows/CHANGELOG.md:

  • Removed redundant #### Desktop (Windows) subheadings across all versions for symmetry with the other single-platform packages (file_picker_darwin, file_picker_linux, file_picker_android, file_picker_web).