I had the same concern and posted on reddit about it long ago before deleting my reddit. Basically chose github at sign up as I didn't even know what OIDC was at the time. Then built a whole network around it with github as a single point of failure. There's no way to migrate it to another sign in method. Email sign up is not offered. The reasoning from Tailscale is that then they need to manage the whole extra user identification thing as well.
Anyway this is old but the solution is either tear down the network and start again with OIDC (can use codeberg for that). Or if like me that means reconfiguring 40 docker containers and a whole network built around the magic dns there is another option.
You need to login and add another user and give that user admin rights as well. I ended up adding that and then setting it up so they can sign in using yubikey. Tailscale recommend doing this regardless.