Update vcpkg.json - lock flatbuffer version (#23046)
### Description
Locking version introduced in:
https://github.com/microsoft/onnxruntime/blob/03ea5dc495bfb48977d23705fa7cf184866a9f7f/onnxruntime/core/flatbuffers/schema/ort_training_checkpoint.fbs.h#L11-L13
### Motivation and Context
Resolve issue for version `>=1.20.`
https://github.com/microsoft/onnxruntime/issues/22666
