Update provider protocol to V6

[jharley]

The provider currently is built against the V5 protocol supporting clients as far back as Terraform 0.12.0.

There is tooling available to translate the V5 resources written in the Plugin SDK to V6 (link).

The V6 Protocol is the current standard and moves the minimum supported version of Terraform to 1.0 (released June, 2021), but this will drop support for all previous clients. Telemetry shows that we have about two folks using a pre-1.0 version of Terraform but also with a very old release of the provider.