libsnapshot: Reject bad cow versions.

Remove CowWriter::GetCowVersion. Instead, reject any update that asks
for VABC but has an unsupported version field. Do not fallback to legacy
VAB as the update was likely built improperly.

Bug: 280529365
Test: vts_libsnapshot_test
Change-Id: Ibc0f981801fd47bf39d7a19944134e4b3c66e5bf