Remove terraform-plugin-sdk/v2 Dependency

### terraform-plugin-testing version

```
1.5.1
```

### Use cases

This issue is a placeholder for removing the `github.com/hashicorp/terraform-plugin-sdk/v2` dependency. The `terraform-plugin-testing` Go module was created with the explicit goal of being an easy migration from using the equivalent testing functionality of the `github.com/hashicorp/terraform-plugin-sdk` Go module, but that meant keeping compatibility to support `helper/resource` package `TestCase` and `TestStep` type `Providers` and `ProviderFactories` fields which directly implement sdk/v2 types. This sometimes indirect dependency on sdk/v2 can cause issues like #185.

### Proposal

Those fields, and any other potential usage of sdk/v2 types, should be deprecated in a 1.x release with the goal of removing the dependency in the 2.0.0 release of this Go module.

### References

- #185