Update authentication section to reflect OAuth server to server credentials

[kwin]

The page at https://developer.adobe.com/experience-cloud/cloud-manager/guides/getting-started/authentication/ at least needs to be updated to reflect the new way of server 2 server authentication (https://developer.adobe.com/developer-console/docs/guides/authentication/ServerToServerAuthentication/implementation/#rotating-client-secrets). JWT service authentication is deprecated.