Set the minimum network protocol version to `170_050` for NU5 testnet

## Motivation

On testnet, Zebra should always reject pre-NU5 nodes.

### Scheduling

We need to wait for:
- NU5 testnet activation - around April 22
- Enough NU5 nodes to be deployed on testnet

## Tasks

- Update the `INITIAL_MIN_NETWORK_PROTOCOL_VERSION` to `Testnet, Nu5`