Resolve version correctly for backend schema

[radeksimko]

This is to address:

• terraform_remote_state config incorrectly flagged with errors vscode-terraform#1580

Depends on:

• schema: Replace CoreModuleSchemaForConstraint w/ ResolveVersion terraform-schema#275

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This PR changes the meaning of the version in the snippet of code being changed. Previously it referred to the discovered/installed Terraform version, whether or not we had the schema for it. With this patch, the meaning changes to "language/schema version".

In practice though it remains the same for the likely majority of cases (any stable version between 0.12 and 1.6) but not for the other cases, where we fall back on either 0.12.0 or latest known version (currently 1.6.0).

Importantly, this does not impact the discovered/installed version we persist and make available, e.g. here

Also, the new logic inside ResolveVersion reflects the way we handled various edge cases in the previous implementation, as shown in the attached test in hashicorp/terraform-schema#275

[dbanck]

I like this approach

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