Log SAML session_expires_at (SessionNotOnOrAfter)

What does this MR do and why?

Log SAML session_expires_at (SessionNotOnOrAfter) to gain visibility when troubleshooting user SAML authentications.

Changelog: changed

References

• SAML SSO support session timeout (!194914 - merged)
• Introduced new FF for group_saml (!197293 - merged)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Configure SAML
2. SAML Authenticate
3. Review entries in gitlab/gitlab-rails/auth_json.log for the presence of session_expires_at

MR acceptance checklist

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