resource_aws_elasticache_replication_group: Add arn attribute

[angelabad]

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Closes #15267

Release note for CHANGELOG:

resource_aws_elasticache_replication_group: Add arn attribute

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccAWSElasticacheReplicationGroup_vpc'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -count 1 -parallel 20 -run=TestAccAWSElasticacheReplicationGroup_vpc -timeout 120m
=== RUN   TestAccAWSElasticacheReplicationGroup_vpc
=== PAUSE TestAccAWSElasticacheReplicationGroup_vpc
=== CONT  TestAccAWSElasticacheReplicationGroup_vpc
--- PASS: TestAccAWSElasticacheReplicationGroup_vpc (601.58s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	601.621s
...

[gdavison]

Thanks for the PR, @angelabad. LGTM 🚀

--- PASS: TestAccAWSElasticacheReplicationGroup_basic (859.50s)
--- PASS: TestAccDataSourceAwsElasticacheReplicationGroup_basic (937.03s)

[ghost]

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