Replace CircleCI with GitHub actions

[dbanck]

While doing some feature work, I noticed that terraform-json is still using CircleCI. The integration stopped working a while ago (for known reasons), so I migrated everything to GHA.

I made some minor changes in the process:

• Created a test matrix for all supported Go versions
  • We can't use anything < 1.16 (yet), because the tests rely on ReadDir on os.File
  • We can't test Windows (yet), because of newlines
• Use go test instead of gotestsum to run the tests

TODO

• Disable CircleCI integration

[radeksimko]

We can't use anything < 1.16 (yet), because the tests rely on ReadDir on os.File

This seems 👌🏻

We can't test Windows (yet), because of newlines

That sounds fixable, but we can do that in a separate PR later.

[radeksimko]

btw. I disabled the CircleCI webhook in the repo.