GitHub's issuing CA has expired (DigiCert High Assurance EV Root CA) (i.imgur.com)

Unfortunately, I'm still having an issue with this.

...if I only knew more about computers. :(

That's actually the cert on your computer allowing you to trust it that has expired. https://blog.digicert.com/expired-intermediate-certificate/

Seeing this on multiple sites which use DigiCert for SSL provider.

Github, Bitbucket, Heroku, and even DigiCert's own site.