collab: Make `github_user_login` required in `LlmTokenClaims` (#18882)

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