docs: describe the current format restrictions

[Adel-Ayoub]

The README now covers the policy argument on vault create and open, the disabled-format result the archive and stream methods return, and file hashing in place of the old streaming example.

Streaming encryption and .mvex archives are marked withdrawn rather than deleted, since both shipped and are meant to return over authenticated formats.

What to change

environment:
  sdk: ^3.10.8
  flutter: '>=3.3.0'    # before
  flutter: '>=3.38.9'   # after

The Dart constraint in the same file already excluded anything below roughly 3.38, so the old floor was dead text.