Bump Terraform SDK dependency to 0.12.6

[Didainius]

This provider is currently using 0.12.0 version of Terraform SDK. The latest one is 0.12.6 and it has a few sdk bugfixes which may be relevant:

Introduced in 0.12.4:

• plugin/sdk: Prevent empty strings from being replaced with default values (prevent an empty string from being lost hashicorp/terraform#21806)
• plugin/sdk: Ensure resource timeouts are not lost when there is an empty plan (Private data and timeouts were lost in empty plans hashicorp/terraform#21814)
• plugin/sdk: Don't add null elements to diagnostic paths when validating config (don't append nil elements to a diagnostic cty.Path hashicorp/terraform#21884)

Introduced in 0.12.6:

• plugin/sdk: allow MinItems > 1 when dynamic blocks (MinItems with dynamic blocks hashicorp/terraform#22221)
• plugin/sdk: fix reflect panics in helper/schema validation (don't reflect nil in schema validation hashicorp/terraform#22236)

Both acceptance tests and binary tests pass.

[dataclouder]

Acceptance tests pass.
Binary tests pass with both terraform 0.12.0 and 0.12.6

[lvirbalas]

Thank for doing this.