Revert having allowed values for add_to_collaborators

[jodelasur]

We are currently having issues with v2.2.0 when using read and write as permission values for Repository.add_to_collaborators. With #1996, only specific permission values are accepted. While the docs do give a list of values for permission, it also says that we "can also specify a custom repository role name, if the owning organization has defined any".

https://docs.github.com/en/rest/collaborators/collaborators?apiVersion=2022-11-28#add-a-repository-collaborator