Remove support for Client Secret

[frederikprijck]

Changes

With ClientSecret being marked as obsolete for 4 years now, we are removing this.

References

• https://github.com/auth0/auth0-oidc-client-net/pull/112

Checklist

• [x] I have read the Auth0 general contribution guidelines
• [x] I have read the Auth0 Code of Conduct
• [x] All existing and new tests complete without errors
• [x] All code guidelines in the CONTRIBUTING documentation have been run/followed