The GitHubAppCredentials converter does not support apiUri.

Note: This makes this very unreliable. In the context of a Multibranch / Organization scan and webhook events, it does not seem to matter and the credentials works. However in the context of a pipeline execution that needs to generate the token (typically a checkout step) the default endpoint https://api.github.com/ is used and that cause a failure. There is a confusing behavior that as long as the cached token is valid (about an hour), everything should work fine...