mcs, tso: keyspace-group split-range can't split default keyspace group

[binshi-bing]

Enhancement Task

root@serverless-cluster-pd-0:/# ./pd-ctl keyspace-group split-range 0 2 1 5000
Failed! [500] "cannot modify default keyspace's state"

for _, keyspace := range old {
	**if keyspace == utils.DefaultKeyspaceID {
		return nil, nil, ErrModifyDefaultKeyspace
	}**
	if startKeyspaceID <= keyspace && keyspace <= endKeyspaceID {
		newSplit = append(newSplit, keyspace)
		newKeyspaceMap[keyspace] = struct{}{}
	}
}