Elixir: Fix negative steps

Fix the following warnings

```
warning: negative steps are not supported in String.slice/2, pass 14..-1//1 instead
  (elixir 1.17.3) lib/string.ex:2404: String.slice/2
  test/elixir/test/uuids_test.exs:91: anonymous fn/2 in UUIDsTest."test utc_id uuids are correct"/1
  (elixir 1.17.3) lib/enum.ex:2531: Enum."-reduce/2-lists^foldl/2-1-"/3
  test/elixir/test/uuids_test.exs:89: UUIDsTest."test utc_id uuids are correct"/1
  (ex_unit 1.17.3) lib/ex_unit/runner.ex:485: ExUnit.Runner.exec_test/2
  (stdlib 4.3.1.5) timer.erl:235: :timer.tc/1
  (ex_unit 1.17.3) lib/ex_unit/runner.ex:407: anonymous fn/6 in ExUnit.Runner.spawn_test_monitor/4
```

Related PR: #5372
Related issue: #5386

Author: jiahuili430

test/elixir/test/uuids_test.exs

@@ -88,7 +88,7 @@ defmodule UUIDsTest do
 
     Enum.reduce(resp.body["uuids"], fn curr, acc ->
       assert String.length(curr) == 14 + String.length(@utc_id_suffix)
-      assert String.slice(curr, 14..-1//-1) == @utc_id_suffix
+      assert String.slice(curr, 14..-1//1) == @utc_id_suffix
       assert curr > acc
       curr
     end)