Fix issue where terminating gateway service resolvers weren't properly cleaned up

agent/proxycfg/terminating_gateway.go

@@ -354,8 +354,13 @@ func (s *handlerTerminatingGateway) handleUpdate(ctx context.Context, u UpdateEv
 		// There should only ever be one entry for a service resolver within a namespace
 		if resolver, ok := resp.Entry.(*structs.ServiceResolverConfigEntry); ok {
 			snap.TerminatingGateway.ServiceResolvers[sn] = resolver
+			snap.TerminatingGateway.ServiceResolversSet[sn] = true
+		} else {
+			// we likely have a deleted service resolver, and our cast is a nil
+			// cast, so clear this out
+			delete(snap.TerminatingGateway.ServiceResolvers, sn)
+			snap.TerminatingGateway.ServiceResolversSet[sn] = false
 		}
-		snap.TerminatingGateway.ServiceResolversSet[sn] = true
 
 	case strings.HasPrefix(u.CorrelationID, serviceIntentionsIDPrefix):
 		resp, ok := u.Result.(structs.Intentions)