etcdserver: add ConfChangeAddLearnerNode to the list of config changes

To fix a panic that happens when force-new-cluster flag is passed to
etcd node if the cluster had learner nodes added from before

Fixes #12285

etcdserver/raft.go

@@ -618,6 +618,7 @@ func restartAsStandaloneNode(cfg ServerConfig, snapshot *raftpb.Snapshot) (types
 // ID-related entry:
 // - ConfChangeAddNode, in which case the contained ID will be added into the set.
 // - ConfChangeRemoveNode, in which case the contained ID will be removed from the set.
+// - ConfChangeAddLearnerNode, in which the contained ID will be added into the set.
 func getIDs(lg *zap.Logger, snap *raftpb.Snapshot, ents []raftpb.Entry) []uint64 {
 	ids := make(map[uint64]bool)
 	if snap != nil {
@@ -632,6 +633,8 @@ func getIDs(lg *zap.Logger, snap *raftpb.Snapshot, ents []raftpb.Entry) []uint64
 		var cc raftpb.ConfChange
 		pbutil.MustUnmarshal(&cc, e.Data)
 		switch cc.Type {
+		case raftpb.ConfChangeAddLearnerNode:
+			ids[cc.NodeID] = true
 		case raftpb.ConfChangeAddNode:
 			ids[cc.NodeID] = true
 		case raftpb.ConfChangeRemoveNode: