upgrading CI ?

[asmodehn]

Hi,

I regularly use committee to help me with formatting, so I decided to have a go at updating the CI (I saw it is still using old elixir versions, with old source formatting).

It may be a minor change, but in case you want to have a look at it, I have a working workflow for elixir 1.12, 1.13 and 1.14 there.

I also changed my default branch to main, but I am not sure if it is something you'd want to do here ?

Anyway, let me know if you want a specific PR related to some of these changes in this repo.

Cheers !