Amendment for Subcoin milestone3 (#2376)

In light of recent developments, it has become evident that fully syncing
to the tip of the Bitcoin network and enabling new nodes to perform fast
sync to the latest Bitcoin state is more challenging than initially anticipated,
caused by the huge state of UTXO set (over 12GiB). As a result, I propose
adjusting the delivery goal for this milestone.

The most significant known blocker is paritytech/polkadot-sdk#4.
Other underlying issues may also contribute to the difficulty. Recent experiments
have shown that fast sync from around block height 580,000 is currently infeasible,
succeeding only on machines with 128GiB of memory (paritytech/polkadot-sdk#5053 (comment)),
which is impractical for most users. Nevertheless, we have successfully demonstrated that
decentralized fast sync is possible within a prototype implementation.

While syncing to the Bitcoin network's tip remains a future target, addressing
the existing technical challenges will require substantial R&D efforts.
We remain committed to exploring potential solutions, including architectural
changes and contributing to resolving issue paritytech/polkadot-sdk#4,

applications/Subcoin.md

@@ -129,7 +129,7 @@ This project is planned as 3 milestones, completed in three months.
 | 0c. | Testing and Testing Guide | Core functions will be fully covered by comprehensive unit tests to ensure functionality and robustness. In the guide, we will describe how to run these tests. |
 | 0d. | Docker | We will provide a Dockerfile(s) that can be used to test all the functionality delivered with this milestone. |
 | 0e. |	Article | We will publish an article that explains what was done/achieved as part of the grant. |
-| 1. | Fast sync | A bootstrap node will be deployed so that the new subcoin nodes can sync to the tip of the Bitcoin network very quickly using the built-in advanced syncing strategy (e.g., light state sync) in Substrate. |
+| 1. | Fast sync | A bootstrap node will be deployed to demonstrate that the new subcoin nodes can perform a fast sync from other subcoin nodes using the built-in advanced syncing strategy (e.g., light state sync) in Substrate. |
 | 2 | Bitcoin Transaction Broadcasting | We will add an RPC call for broadcasting a Bitcoin transaction to the Bitcoin network from polkadot.js.org. |
 
 ## Future Plans