Fix NotFound not detected during a read on aws sns platform applications

[MagicMicky]

We've seen some issues with terraform where the sns platform application has been created via Terraform, but deleted via another mean (ie: webui).
Terraform wouldn't detect that the SNS platform application has been deleted and would error out. To fix this we had to manually remove the impacted resources from the state.

Error: Error refreshing state: 2 error(s) occurred:

* module.redacted.aws_sns_platform_application.my_apn1: 1 error(s) occurred:

* module.redacted.aws_sns_platform_application.my_apn1: aws_sns_platform_application.my_apn1: NotFound: PlatformApplication does not exist
    status code: 404, request id: <redacted>
* module.redacted.aws_sns_platform_application.my_apn2: 1 error(s) occurred:

* module.redacted.aws_sns_platform_application.my_apn2: aws_sns_platform_application.my_apn2: NotFound: PlatformApplication does not exist
    status code: 404, request id: <redacted>

This error was already present on other resources, but have been fixed a while ago : hashicorp/terraform#4891
Code can now be found at :
https://github.com/terraform-providers/terraform-provider-aws/blob/20151d7b332763952f44209e7356ebb661d9add0/aws/resource_aws_sns_topic.go#L198-L202

Looking at how it is done for the SNS topics, I figured it wouldn't be super difficult to fix and could make a small PR to help.

Tests are running fine

make test                                                                                                                                                                                                 bugfix-snsplatformapp-notfound
==> Checking that code complies with gofmt requirements...
go test ./... -timeout=30s -parallel=4
?   	github.com/terraform-providers/terraform-provider-aws	[no test files]
ok  	github.com/terraform-providers/terraform-provider-aws/aws	1.994s

[bflad]

LGTM, thanks @MagicMicky! 🚀

[bflad]

This has been released in version 1.44.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

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