Migrate CircleCI workflows to GitHub actions

[mao-sz]

Because

There is a CircleCI config set up but this doesn't seem to actually be used for any PRs affecting relevant files. In a similar vein to TheOdinProject/odin-bot-v2#648, it'd be easier to make use of GH actions since it's integrated and we get unlimited action credits as well.

We only really need the workflow to run on changes to files in the solution dirs, and only run solution tests (since exercise tests suites will likely fail and skip many tests).

Caveats

• Runs all solution tests, not just for the changed solution dirs (seemed a little unnecessarily complex to configure, and running all solution tests is a negligible impact anyway).
• Won't check to keep tests for exercise and solution in sync - got to do this manually. Can't think of a reasonable workflow that can ensure this (has many caveats of its own).

This PR

• Adds GH workflow to run solution tests on changes to any solution dir
• Removes unused CircleCI config

Issue

N/A

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. 01_helloWorld: Update test cases
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If this PR includes any changes that affect the solution of an exercise, I've also updated the solution in the /solutions folder

[mao-sz]

Test PR demonstrating workflow in action: mao-sz#1

[KevinMulhern]

Looks great to me 🚀