TMP use ELixir 1.15.8 in CI

Elixir 1.15 is the first Elixir version series to support Erlang 26

`mix_task_archive_deps` uses `Mix.Dep.load_on_environment/1` which was
removed in Elixir 1.16.0.

.github/workflows/action.yml

@@ -8,7 +8,7 @@ jobs:
         erlang:
           - '26.2'
         elixir:
-          - '1.16.3'
+          - '1.15.8'
         rmqref:
           - v3.13.x
     steps: