resource/cloudfront_distribution: Use flattenStringSet helper

[mattburgess]

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Relates #6867

Release note for CHANGELOG:

NONE

Output from acceptance testing:

$ make testacc TEST=./aws TESTARGS='-run=TestAccAWSDataSourceCloudFrontDistribution'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -count 1 -parallel 20 -run=TestAccAWSDataSourceCloudFrontDistribution -timeout 120m
=== RUN   TestAccAWSDataSourceCloudFrontDistribution_basic
=== PAUSE TestAccAWSDataSourceCloudFrontDistribution_basic
=== CONT  TestAccAWSDataSourceCloudFrontDistribution_basic
--- PASS: TestAccAWSDataSourceCloudFrontDistribution_basic (349.97s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	350.003s

[anGie44]

LGTM, thanks @mattburgess 🚀

[ghost]

This has been released in version 3.20.0 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!