data-source/aws_elb: Export arn attribute

[115100]

The documentation says that data source for aws_elb shall return the
same attributes as the resource type.

This commit exports arn on the data source to match the documented
behaviour.

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Release note for CHANGELOG:

data-source/aws_elb: Export arn attribute

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccDataSourceAWSELB_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./... -v -count 1 -parallel 20 -run=TestAccDataSourceAWSELB_basic -timeout 120m
?       github.com/terraform-providers/terraform-provider-aws   [no test files]
=== RUN   TestAccDataSourceAWSELB_basic
=== PAUSE TestAccDataSourceAWSELB_basic
=== CONT  TestAccDataSourceAWSELB_basic
--- PASS: TestAccDataSourceAWSELB_basic (52.27s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws       52.307s
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws/internal/flatmap      0.002s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws/internal/keyvaluetags 0.008s [no tests to run]

[bflad]

Looks good, thanks @115100 🚀

Output from acceptance testing:

--- PASS: TestAccDataSourceAWSELB_basic (13.85s)

[ghost]

This has been released in version 2.43.0 of the Terraform AWS 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 for triage. Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!