Elixir verison in Mix.exs might be too high

Hi!

I noticed that the mix.exs Elixir version was bumped to 1.16, but I'm not sure that Retry uses any features that require it.

Was this change made simply because the test matrix/local work was being updated off earlier versions? If so, I'd recommend lowering the version so that warnings aren't emitted for consumers that are on lower Elixir versions.

Thanks!