Handle gone gracefully in read for azurerm_mssql_server_dns_alias

[turkenh]

This PR handles manual resource deletion gracefully for azurerm_mssql_server_dns_alias resource as mentioned here:

https://learn.hashicorp.com/tutorials/terraform/provider-setup#implement-read

When you create something in Terraform but delete it manually, Terraform should gracefully handle it. If the API returns an error when the resource doesn't exist, the read function should check to see if the resource is available first. If the resource isn't available, the function should set the ID to an empty string so Terraform "destroys" the resource in state. The following code snippet is an example of how this can be implemented; you do not need to add this to your configuration for this tutorial.

Fixes #18614

[katbyte]

Thanks @turkenh - LGTM 🌵

[github-actions]

This functionality has been released in v3.26.0 of the Terraform Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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