[BUG]: GitHub_organization_custom_properties

### Expected Behavior

If I want to create a custom property with mutli_select (organization wide) I cannot set mutiple default values.

```hcl
resource "github_organization_custom_properties" "repo_props" {
property_name = "temp1"
value_type = "multi_select"
required = true
description = "BlaBla"
allowed_values = [
"b1",
"b2",
"b3"
]
default_value = ["b1", "b2"] # does not work because terraform expects a string here but would work with the api
default_value = "b1,b2" # does not work as GitHub expects a array here.
}
```

It's therefore not possible to set 2 default values as far as I see as there is a mismatch between terraform and the GitHub API.

### Actual Behavior

I think I should be able to set a list or a string here... string for single select and list for multi select

### Terraform Version

Version 6.7

### Affected Resource(s)

github_organization_custom_properties

### Terraform Configuration Files

```hcl

```

### Steps to Reproduce

_No response_

### Debug Output

```shell

```

### Panic Output

```shell

```

### Code of Conduct

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