If your GitHub server is self-hosted (on-prem or in private cloud) and using a self-signed certificate, you’ll need to make sure the certificate installed on your GitHub server is in your JVM trust store. This can easily be obtained following the post I mentioned earlier (just substitute Gitlab with your GitHub server)
If you’re using GitHub.com, it would be strange for your JVM to not trust GitHub.com by default, since it uses a cert signed by trusted CAs. However, if that’s the case, it might point to another problem so don’t hesitate to clarify!
Which ones have you tried so far?