Upgrade to new Minitest constant + new dev env + 3.3 bugfix

[davetron5000]

Problem

A few:

• Minitest changed from MiniTest to Minitest like 10 years ago, but compatibilty was removed somewhat recently. Discovered by @rsanheim in

  323

• Scaffold test failed on Ruby 3.3
• Build matrix in CircleCI was out of date, plus Circle doesn't really work and was failing due to their ever-changing YAML configuration nonsense, which I absolutely despise.
• Dev environment is kinda nonexistent.

Solution

• Update build matrix to only the supported versions of Ruby. At this time, that is 3.1, 3.2, and 3.3
• Update dev environment tooling to allow easily running GLI's tests on all three using Docker (see CONTRIBUTING.md for docs).
• Change MiniTest to Minitest to avoid having to deal with compatibility. That change is 10 years old, so even super old unsupported Rubies should still work.
• Fix bug in scaffolded Gemspec discovered on Ruby 3.3. Have to use Gem::Specification.load to load the spec into the Rakefile.
• Remove CircleCI. The replacement is that I will just run the tests. Sadly, this is a far better and more reliable way of doing things than Circle CI. I do not wish them well.

[davetron5000]

Fixes #323

[davetron5000]

Released as 2.21.2