fix incorrect use of sort.StringSlice

A newer version of staticheck found these:

> SA4029: sort.StringSlice is a type, not a function, and
> sort.StringSlice(variants) doesn't sort your values; consider using
> sort.Strings instead (staticcheck)

endpoint/endpoint_zfs_abstraction.go

@@ -168,7 +168,7 @@ func (s AbstractionTypeSet) String() string {
 	for i := range s {
 		sts = append(sts, string(i))
 	}
-	sts = sort.StringSlice(sts)
+	sort.Strings(sts)
 	return strings.Join(sts, ",")
 }