`Custom scopes are not allowed for this request` when using `PrivateKey`

Describe the bug?

It worked in 7.0.6 but doesn't work in all versions between 8.0.0 and 9.0.0.

When trying to use any API with PrivateKey, the following exception is thrown:

Okta.Sdk.Client.ApiException: Error calling GetBearerToken: {"error":"invalid_scope","error_description":"Custom scopes are not allowed for this request."}
    at Okta.Sdk.Api.OAuthApi.GetBearerTokenWithHttpInfoAsync(CancellationToken cancellationToken)
   at Okta.Sdk.Api.OAuthApi.GetBearerTokenAsync(CancellationToken cancellationToken)
   at Okta.Sdk.Client.DefaultOAuthTokenProvider.RequestAccessTokenAsync(CancellationToken cancellationToken)
   at Okta.Sdk.Client.DefaultOAuthTokenProvider.GetAccessTokenResponseAsync(Boolean forceRenew, CancellationToken cancellationToken)
   at Okta.Sdk.Client.DefaultOAuthTokenProvider.AddOrUpdateAuthorizationHeader(RequestOptions requestOptions, String requestUri, String httpMethod, CancellationToken cancellationToken)
   at Okta.Sdk.Api.ApplicationApi.GetApplicationWithHttpInfoAsync(String appId, String expand, CancellationToken cancellationToken)
   at Okta.Sdk.Api.ApplicationApi.GetApplicationAsync(String appId, String expand, CancellationToken cancellationToken)
   at TestProject2.UnitTest1.TestMethod1() in C:\Users\Artur\source\repos\TestProject2\UnitTest1.cs:line 21

What is expected to happen?

The API call should succeed.

What is the actual behavior?

ApiException: Error calling GetBearerToken: {"error":"invalid_scope","error_description":"Custom scopes are not allowed for this request."}

Reproduction Steps?

Add config to appsetting.json

{
  "Okta": {
    "Client": {
      "OktaDomain": "xxx",
      "ClientId": "xxx",
      "AuthorizationMode": "PrivateKey",
      "PrivateKey": {
        "p": "xxx",
        "kty": "RSA",
        "q": "xxx",
        "d": "xxx",
        "e": "xxx",
        "use": "sig",
        "qi": "xxx",
        "dp": "xxx",
        "alg": "RS256",
        "dq": "xxx",
        "n": "xxx"
      },
      "Scopes": [
        "okta.apps.manage"
      ]
    }
  }
}

Try getting the application

var api = new ApplicationApi();
var app = await api.GetApplicationAsync("app id");

Additional Information?

No response

.NET Version

8.0.403

SDK Version

9.0.0

OS version

BuildNumber	Caption	OSArchitecture	Version
19045	Microsoft Windows 10 Enterprise	64-bit	10.0.19045

okta / okta-sdk-dotnet

`Custom scopes are not allowed for this request` when using `PrivateKey` #753