Add two missing states for RDS cluster update

[Kdecherf]

Superceded by #17111

Enabling/disabling IAM auth on an existing RDS cluster led to a fail of
terraform apply as 'configuring-iam-database-auth' was not an expected pending
state.

I also identified another missing pending state: 'renaming'.

This is my first PR ever on a terraform-related project, if things are missing in my PR please let me know.

As I'm not confident with the test codebase, I'm not sure if there is a specific test case to update or add for what I'm changing.

Here is a snippet to reproduce the issue, set iam_database_authentication_enabled to true after the initial creation:

terraform {
  required_providers {
    aws = {
      source = "hashicorp/aws"
    }
  }
  required_version = ">= 0.13"
}

provider "aws" {
  region = "eu-west-1"
}

resource "aws_rds_cluster" "cluster" {
  cluster_identifier = "cluster-test"

  engine = "aurora-postgresql"
  engine_version = "11.6"
  deletion_protection = false
  backup_retention_period = 1
  skip_final_snapshot = true
  master_username = "root"
  master_password = "whatever"
  apply_immediately = true

  iam_database_authentication_enabled = false
}

resource "aws_rds_cluster_instance" "instance" {
  cluster_identifier = aws_rds_cluster.cluster.id
  identifier = "cluster-test-instance"
  instance_class = "db.t3.medium"
  engine = "aurora-postgresql"
  engine_version = "11.6"
}

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Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'

...

[github-actions]

Welcome @Kdecherf 👋

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 CONTRIBUTING 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! 😃

[YakDriver]

Thank you for your work on this! Your commits were included with #17111. Of course, you still receive credit for them.

[github-actions]

This functionality has been released in v3.63.0 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. Thank you!

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.