terraform-provider-null: Update to 3.0.0 and switch to terraform/provider.mk

pkgsrc changes:
- According changelog the only incompatibility is with Terraform 0.11.x so it
  should be safe to bump it to the latest 2.x version
- Adapt to terraform/provider.mk so that the provider can be used with
  Terraform and OpenTofu.
  Please note that now it no longer depends on terraform or opentofu so
  the desired version should be manually installed.


Changes:
3.0.0
Binary releases of this provider now include the linux-arm64 platform.

BREAKING CHANGES:
- Upgrade to version 2 of the Terraform Plugin SDK, which drops
  support for Terraform 0.11. This provider will continue to work as
  expected for users of Terraform 0.11, which will not download the
  new version.