Fetch the fee faucet id from the latest stored block header instead of the genesis

Originally discussed in [this comment](https://github.com/0xMiden/faucet/pull/262#discussion_r3769759885).

After https://github.com/0xMiden/rust-sdk/pull/2397 was merged, we should update the [`fetch_fee_faucet_id`](https://github.com/0xMiden/faucet/blob/5bc853758e0468176a9b9edb5997b5cb1d93e021/crates/faucet/src/lib.rs#L978) function so that it calls `Client::get_latest_block_header` instead of `Client::get_block_header_by_number`.