Elixir 1.15

[taylordowns2000]

This should fix that annoying Elixir-LS issue. (I've tried to trigger it a few times and so far so good.) Also, everything appears to work well on Elixir 1.15 and Erlang 26.

Update

1. Elixir 1.15 jeremyjh/dialyxir#512 - when this is merged we'll get cleaner dialyzer output
2. there are a handful of dialyzer errors - should we be worried about these?

Review checklist

• I have performed a self-review of my code
• I have verified that all appropriate authorization policies have been implemented and tested
• If needed, I have updated the changelog
• Product has QA'd this feature

[stuartc]

Thanks for this @taylordowns2000, re: the dialyzer warnings - they are fine for now (i.e. not functional difference) but we will have to address them. I gave it a quick go around, and it looks like %Task{ref: reference()} is marked as opaque now, which means we're not supposed to directly reference it.

The two modules that have this issue will need to be refactored a bit, will have to schedule that in.

[stuartc]

Changing this to 1.15.4 since it looks like it's based on OTP26 already: https://github.com/erlef/docker-elixir/blob/5ac15c1d2bbdbf051f198e471c93f604f71d87eb/1.15/Dockerfile#L1