chore: use josecfreittas/elixir-coverage-feedback-action in ci

[Sed9yDataBank]

Context

To check for tests coverage, the action uses Elixir's built-in mix test --cover so less overhead than using coveralls.io 📝 https://github.com/marketplace/actions/elixir-coverage

[github-actions]

Tests summary

Finished in 0.3 seconds (0.3s async, 0.01s sync)
Running ExUnit with seed: 841833, max_cases: 8

✅ 0 failures (39 tests)
✅ 84.42% coverage (80% is the minimum)

Coverage details

Percentage | Module
-----------|--------------------------
    45.45% | ExLinear.Config
    84.38% | ExLinear.Client
   100.00% | ExLinear
   100.00% | ExLinear.Issue
   100.00% | ExLinear.Issue.CreateInput
   100.00% | ExLinear.Issue.UpdateInput
-----------|--------------------------
    84.42% | Total