chainreg: accommodate buried taproot deployment in Bitcoin Core v31

Removes "taproot" from deployments:
bitcoin/bitcoin#26201

Adds "script_flags" field to RPC getdeploymentinfo:
bitcoin/bitcoin#32998

Author: pinheadmz

chainreg/taproot_check.go

@@ -2,6 +2,7 @@ package chainreg
 
 import (
 	"encoding/json"
+	"slices"
 
 	"github.com/btcsuite/btcd/rpcclient"
 )
@@ -48,6 +49,7 @@ func backendSupportsTaproot(rpc *rpcclient.Client) bool {
 	}
 
 	info := struct {
+		ScriptFlags []string `json:"script_flags"`
 		Deployments map[string]struct {
 			Type   string `json:"type"`
 			Active bool   `json:"active"`
@@ -59,6 +61,14 @@ func backendSupportsTaproot(rpc *rpcclient.Client) bool {
 		return false
 	}
 
+	// Before Bitcoin Core v31, taproot was still included as a BIP9
+	// deployment.
 	_, ok := info.Deployments["taproot"]
-	return ok
+
+	// Since v31, taproot is activated at genesis and no longer appears
+	// as a deployment. Also in v31, Bitcoin Core added a "script_flags"
+	// field to getdeploymentinfo which lists all the verification flags.
+	hasFlag := slices.Contains(info.ScriptFlags, "TAPROOT")
+
+	return ok || hasFlag
 }