feat: enable testnet4 support in the bitcoin adapter

[mihailjianu1]

This involved:

• changing the seeds,
• upgrading the bitcoin crate (see base branch)
• finally, fixing a bug in the header validation, a fix which is required in the bitcoin canister too (and even on the bitcoin crate if we want to use their helpers): For testnet networks, the difficulty temporarily drops to the minimum if no block has been found in 20 minutes. Block 30239 is an example of such block, and it (coincidentally) occurs right before a "difficulty change", once every 2016 blocks. With the previous implementation, this leads to the difficulty to be dropped massively (to the minimum) on the next difficulty batch, because the difficulty of an "epoch" block (in this case block 30240) depends directly on its previous block (block 30239, which had the "temporarily" drop in difficulty). The fix involved making the new difficulty depend on the difficulty of the previous "epoch block", which does not have a drop in difficulty.