CI updates

[greg-rychlewski]

Changes:

• I updated the CI to use Elixir 1.15 but what happened was it broke some of the migration tests. They were using @moduletag direction: :forward | :backward to run them properly and it seemed like it was relying on the ordering of the lines in the test file. I changed it to describe tag instead and it worked. Also I didn't use OTP 26 because a bunch of tests broke due to map key orderings. But I can work on fixing that a bit later.
• I'm not sure you'll agree, but I moved the unit tests to github actions because it seemed a bit cleaner to me and I believe Earthly is not really needed unless it's for integration tests? But I can revert this if you prefer the old way

[josevalim]

Beautiful. Agreed on using earthly for integration tests. :)