Deprecate the terraform-provider-metal project

[ocobles]

This project will be deprecated in favor of https://github.com/equinix/terraform-provider-equinix

To consider it fully deprecated all boxes below must be checked (same for original issue created in terraform-provider-packet):

• Migration guide should be prepared https://github.com/equinix/terraform-provider-equinix/blob/master/docs/guides/migration_guide_equinix_metal.md
• transition project to deprecated status #235
  README.md should be updated with a deprecation notice (refer to github.com/packethost/standards)
• GitHub project description and link should refer to the new provider
• Issues should be migrated to the new project (or closed and reopened if valid)
• Terraform Registry should be updated with a deprecation notice (index.md)
• 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 (https://www.terraform.io/plugin/sdkv2/best-practices/deprecations#example-resource-renaming is this available in v1 sdk?)

[ocobles]

https://registry.terraform.io/providers/equinix/metal/3.3.0