Update Terraform versions which are tested against

[skorfmann]

This drops tests against 0.12 in favour of testing against the upcoming 0.15 version. That means with this PR being merged we're testing against 0.13, 0.14, and 0.15-beta.

This depends #591 and will help #584 being merged. There are some provider schema differences with the change from 0.12 to 0.13 which don't seem worthwhile working around (I think it's purely testing related, functionality wise it's not an issue).

I think it generally makes sense for now to track the current and prior major release of Terraform core - plus upcoming versions.

[danieldreier]

Do we check what TF version a user is using? My gut feel here is that we should at least warn people, if not error out, if they are using an older version we don't test against

[skorfmann]

Do we check what TF version a user is using? My gut feel here is that we should at least warn people, if not error out, if they are using an older version we don't test against

That's a good point and yes, we do check for the Terraform version and error out. For prior 0.12 it just didn't work at all, that's why we errored out there. Happy to continue with that strict handling though - so, how about bumping the check up to 0.13 then?

[skorfmann]

Went with a warning only for now - see 2f1cc9a

[github-actions]

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