fix: restrict cluster iam role name prefix length (#150)

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### Motivation

IAM Role name length is between 1 and 64.

The IAM Role name_prefix length is between 1 and 38. Because the suffix
length is 26 which is defined [in the terraform
provider](https://github.com/hashicorp/terraform-plugin-sdk/blob/d829d8ebe2cffabeeb7e65204a060619d25b0159/helper/id/id.go#L27).

### Modifications

- Restrict `${var.cluster_name}-cluster` to 37, since the separator `-`
is 1

### Verifying this change

- For new cluster
- For existing cluster

### Documentation

- [x] `no-need-doc`

main.tf

@@ -253,7 +253,7 @@ module "eks" {
   iam_role_arn                  = try(var.cluster_iam.iam_role_arn, aws_iam_role.cluster[0].arn, null)
   create_iam_role               = try(var.cluster_iam.create_iam_role, true)
   iam_role_use_name_prefix      = try(var.cluster_iam.iam_role_use_name_prefix, true)
-  iam_role_name                 = try(var.cluster_iam.iam_role_name, null)
+  iam_role_name                 = try(var.cluster_iam.iam_role_name, substr("${var.cluster_name}-cluster", 0, 37), null)
   iam_role_path                 = try(var.cluster_iam.iam_role_path, var.iam_path, "/StreamNative/")
   iam_role_permissions_boundary = try(var.cluster_iam.iam_role_permissions_boundary, var.permissions_boundary_arn, null)