chore(deps): Use github.com/decred/dcrd/dcrec/secp256k1/v4 directly

[ryanschneider]

To work around the false positive CVE being detected based on our dependency on github.com/btcsuite/btcd/btcec I went down the rabbit hole and learned that btcec is actually a pretty light wrapper around github.com/decred/dcrd/dcrec/secp256k1, so updated to use that directly to minimize our dependency graph.

However, in doing so I learned that github.com/decred/dcrd/dcrec/secp256k1 didn't actually expose the R and S values until very recently, in fact the functionality was just added in decred/dcrd#3188 but hasn't been released yet, hence the go.mod pointing directly to a commit on https://github.com/decred/dcrd master. Once 1.9.0 is released we can update our dep to point to that instead.

This also has the nice affect of significantly lowering the total deps since we don't need to pull in everything else from btcd's dependency chain.