fix(providers): add issuer to GitHub provider for RFC 9207 compliance (#13410)

GitHub recently implemented RFC 9207 by returning an 'iss' parameter in OAuth callbacks.
The openid-client library validates this parameter unconditionally, breaking authentication
for apps that don't configure an issuer.

This fix adds issuer configuration to both standard GitHub and GitHub Enterprise Server,
ensuring RFC 9207 compatibility.

Fixes: https://github.com/langfuse/langfuse/issues/13091

Co-authored-by: Vinay152003 <vinay152003@users.noreply.github.com>