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)

client/zfsabstractions.go

@@ -57,7 +57,7 @@ func (f *zabsFilterFlags) registerZabsFilterFlags(s *pflag.FlagSet, verb string)
 	for v := range endpoint.AbstractionTypesAll {
 		variants = append(variants, string(v))
 	}
-	variants = sort.StringSlice(variants)
+	sort.Strings(variants)
 	variantsJoined := strings.Join(variants, "|")
 	s.Var(&f.Types, "type", fmt.Sprintf("only %s holds of the specified type [default: all] [comma-separated list of %s]", verb, variantsJoined))
 

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, ",")
 }