[MAINT]: Naming of organization custom properties needs the change (from plural to singular)

### Describe the need

Currently data and resource use `properties` (plural) in the naming, but in reality both work only with a single property.

To follow the naming of other resources and properties naming should be changed to `property` (singular).

Resource [organization_custom_properties](https://registry.terraform.io/providers/integrations/github/latest/docs/resources/organization_custom_properties )
Data [organization_custom_properties](https://registry.terraform.io/providers/integrations/github/latest/docs/data-sources/organization_custom_properties)
both use attribute `property_name ` which is only a single one and both can't work with more than a single custom property.

It probably the use case when data and resource should be renamed to **organization_custom_property** (singular),
plurals need to be marked as deprecated within version `v6` of provider and probably removed in `v7`.

### SDK Version

_No response_

### API Version

_No response_

### Relevant log output

```shell

```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct