Change from Github Actions -> CircleCI

[crimson-knight]

I'm not 100% certain on the steps, but I use CircleCI for my full-time job and I'm FAR more familiar with it and how to make it work.

• PR's only run the test suite against the latest Crystal release
• Master/Main gets run using the latest Crystal release
• Master/Main gets run using the minimum supported Crystal release
• Master/Main gets run using the nightly release from Crystal

Just throwing this out there in case anyone would like to tackle some of this. Don't feel you have to do all of the configurations at once!