go.mod: Update hcsshim to v0.8.21

This version brings in some bug fixes to layer handling. The actual fix isn't
present in the diff as it's not used here, but the Windows shim is built from
the tag present in go.mod, so the fix will be in the Windows shim on a new release
of Containerd if this tag is in.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

vendor/github.com/Microsoft/hcsshim/hnsendpoint.go

@@ -7,6 +7,9 @@ import (
 // HNSEndpoint represents a network endpoint in HNS
 type HNSEndpoint = hns.HNSEndpoint
 
+// HNSEndpointStats represent the stats for an networkendpoint in HNS
+type HNSEndpointStats = hns.EndpointStats
+
 // Namespace represents a Compartment.
 type Namespace = hns.Namespace
 
@@ -108,3 +111,8 @@ func GetHNSEndpointByID(endpointID string) (*HNSEndpoint, error) {
 func GetHNSEndpointByName(endpointName string) (*HNSEndpoint, error) {
 	return hns.GetHNSEndpointByName(endpointName)
 }
+
+// GetHNSEndpointStats gets the endpoint stats by ID
+func GetHNSEndpointStats(endpointName string) (*HNSEndpointStats, error) {
+	return hns.GetHNSEndpointStats(endpointName)
+}