multi: use btcd's btcec/v2 and btcutil modules

This commit was previously split into the following parts to ease
review:
 - 2d746f68: replace imports
 - 4008f0fd: use ecdsa.Signature
 - 849e33d1: remove btcec.S256()
 - b8f6ebbd: use v2 library correctly
 - fa80bca9: bump go modules

healthcheck/go.mod

@@ -4,7 +4,8 @@ go 1.16
 
 require (
 	github.com/btcsuite/btclog v0.0.0-20170628155309-84c8d2346e9f
-	github.com/lightningnetwork/lnd/ticker v1.0.0
+	github.com/lightningnetwork/lnd v0.14.2-beta
+	github.com/lightningnetwork/lnd/ticker v1.1.0
 	github.com/stretchr/testify v1.7.0
-	golang.org/x/sys v0.0.0-20210426080607-c94f62235c83
+	golang.org/x/sys v0.0.0-20210915083310-ed5796bab164
 )