Deprecate the terraform-provider-packet project

[displague]

This project should eventually be archived.

Following https://github.com/packethost/terraform-provider-packet/issues/279, new work should move to the Equinix Metal provider.

Here are some initial thoughts:

• [x] Migration guide should be prepared https://github.com/equinix/terraform-provider-metal/issues/1
• [x] #311 README.md should be updated with a deprecation notice (refer to github.com/packethost/standards)
• [x] GitHub project description and link should refer to the new provider
• [x] Issues should be migrated to the new project (or closed and reopened if valid)
• [x] Terraform Registry should be updated with a deprecation notice
• [x] Terraform resource documentation should be updated with a deprecation notice in support of the new resources
• [x] Terraform provider should include any deprecation messages available through metadata or warning messages (https://www.terraform.io/plugin/sdkv2/best-practices/deprecations#example-resource-renaming is this available in v1 sdk?)

Backporting to this project should be done on a case-by-case basis.

[alice-sowerby]

@displague please could you make some of the simpler changes e.g.

1. Update Readme description to more clearly show that you should go to the Equinix version https://github.com/equinix/terraform-provider-metal.
2. Consider whether we should call it “maintained”. Maybe some kind of EOL status or deprecated tag.
3. Deal with the issues attached to this repo (at least prevent any new ones being raised).

[displague]

Consider whether we should call it “maintained”. Maybe some kind of EOL status or deprecated tag.

https://github.com/packethost/standards/issues/77

[ocobles]

Some of the tasks originally listed are no longer applicable/required. The provider doesn't appear listed in the Terraform registry using the search box and external links to old packet provider redirect to metal provider https://registry.terraform.io/providers/packethost/packet/latest/docs

Therefore, I suggest not putting effort into the tasks listed below and mark this repository as End Of Life instead of deprecated, as we can no longer post releases of this project to the registry. It is worth mentioning that these tasks have been covered in Equinix Metal https://github.com/equinix/terraform-provider-metal/issues/232 so anyone redirected to the Equinix Metal provider will be notified of the deprecation and available migration guides/tools.

• [ ] ~Terraform Registry should be updated with a deprecation notice~
• [ ] ~Terraform resource documentation should be updated with a deprecation notice in support of the new resources~
• [ ] ~Terraform provider should include any deprecation messages available through metadata or warning messages~