Update dependencies, use GitHub Actions

[a-lavis]

What

1. Removes .ruby_version
2. Removes appraisals.
3. Removes CircleCI config.
4. Update gems.
5. Add debug and irb gems to :development group in Gemfile.
6. Add GitHub actions.

Why

1. We can set our ruby version using rbenv on local and GitHub actions on CI.
2. we can test multiple versions using rbenv on local and GitHub actions on CI.
3. This a public repo and the testing setup is simple, might as well just use GitHub actions.
4. Staying up to date is good!
5. These gems are useful for debugging - they aren't required and they are built into Ruby, but by adding them to the Gemfile we can use the latest versions of these tools.
6. We can ensure that tests are passing for a PR before merging it.

Overall, this will make it easier to contribute to mail-x_smtpapi.

[amancevice]

let's change the main branch to main while we're here

[a-lavis]

I'll go ahead and update it in main.yml, but I don't think I can update the default GitHub branch without admin privileges. Would you be able to do that for me?

[amancevice]

I would make these minor versions. Do we really need to test < 3.2?

[a-lavis]

I figured we might as well since the tests pass. We could keep testing < 3.2 as long as the ruby versions are supported and the tests pass. But I'm also happy to remove them if you'd prefer!

[a-lavis]

@amancevice do you know how to get rid of the CircleCI Pipeline CI error? Is there a way we can get CircleCI to stop watching this repo?

[amancevice]

@a-lavis I'm just gonna merge it and take care of the branch and circle issues after.