This PR makes the `github_user_login` field required in the
`LlmTokenClaims`.
We previously added this in
https://github.com/zed-industries/zed/pull/16316 and made it optional
for backwards-compatibility.
It's been more than long enough for all of the previous LLM tokens to
have expired, so we can now make the field required.
Release Notes:
- N/A