Fix RDS instance bug where switching to IO2 from GP needs the allocated storage set

[nhenjes]

Description

When creating an RDS instance, the Allocated Storage param is passed correctly to AWS. When going from, for example, GP3 to IO2 on an existing instance only a change in the Storage Type is detected by the provider and only Storage Type is passed. AWS' API however, requires that both storage type and Allocated Storage are submitted as part of the request

Relations

Closes #36648

References

When running aws rds modify-db-instance --db-instance-identifier <db-name> --storage-type io2 --iops 3000 the following error is thrown

An error occurred (InvalidParameterCombination) when calling the ModifyDBInstance operation: You must specify both the storage size and iops when modifying the storage size or iops on a DB instance that has iops.

When running aws rds modify-db-instance --db-instance-identifier <db-name> --storage-type io2 --iops 3000 --allocated-storage 100

The result is a successful change

Subsequent runs with the same params result in success meaning passing the allocated storage again is perfectly fine

Output from Acceptance Testing

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.22.2 test ./internal/service/rds/... -v -count 1 -parallel 20 -run='TestAccRDSCluster_storageTypeGeneralPurposeToProvisionedIops'  -timeout 360m
=== RUN   TestAccRDSCluster_storageTypeGeneralPurposeToProvisionedIops
=== PAUSE TestAccRDSCluster_storageTypeGeneralPurposeToProvisionedIops
=== CONT  TestAccRDSCluster_storageTypeGeneralPurposeToProvisionedIops
--- PASS: TestAccRDSCluster_storageTypeGeneralPurposeToProvisionedIops (970.05s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/rds	976.063s

[github-actions]

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[github-actions]

Welcome @nhenjes 👋

It looks like this is your first Pull Request submission to the Terraform AWS Provider! If you haven’t already done so please make sure you have checked out our CONTRIBUTOR guide and FAQ to make sure your contribution is adhering to best practice and has all the necessary elements in place for a successful approval.

Also take a look at our FAQ which details how we prioritize Pull Requests for inclusion.

Thanks again, and welcome to the community! 😃

[DrFaust92]

nhenjes Thank you for your contribution, can you also add a test for this?

[nhenjes]

@DrFaust92 I can work on writing a test for this

[nhenjes]

@DrFaust92 I was able to get my test working properly now. The test will provision a cluster using GP3 at 400GB with 12000 provisioned IOPs then updates to IO2 successfully.

Without the bug fix, the same test reports

        Error: updating RDS DB Instance (terraform-20240517150000387300000001): operation error RDS: ModifyDBInstance, https response error StatusCode: 400, RequestID: b15c2fa4-e202-4778-a85f-09f34ce77dcc, api error InvalidParameterCombination: You must specify both the storage size and iops when modifying the storage size or iops on a DB instance that has iops.
        
          with aws_db_instance.test,
          on terraform_plugin_test.tf line 56, in resource "aws_db_instance" "test":
          56: resource "aws_db_instance" "test" {