Update deps (and remove inner_cotton and excoveralls)

[adamu]

This PR performs some maintenance tasks:

1. Updated CI to run on the latest versions of Erlang and Elixir + bump actions versions
2. Updated dependencies
3. Removed unnecessary dependencies

About inner_cotton and excoveralls

I understand the maintainer developed inner_cotton, so this may be controversial. The reason I removed it is because there were some compiler errors with credo and ssl_verify_fun that were fixed by updating their versions, which was easier to do without first updating the upstream inner_cotton dependency. I think more dependencies adds fragility to a project, and this is a very simple library, so would benefit from as few dependencies as possible. In this case, I replaced inner_cotton by using credo and dialyxir directly, and also made the necessary update to ssl_verify_fun.

I removed excoveralls, because the coverage was only 40% anyway, so I think the dependency was adding only a maintenance burden for this project.