scheduler: Raise auth failure log level from DEBUG to WARN for fail2ban compatibility

[abubakarsabir924-cell]

Fixes #1553

When LogLevel is set to "error" in cupsd.conf, authentication
failure messages were logged at CUPSD_LOG_DEBUG level and never
appeared in logs. This prevented fail2ban from detecting failed
login attempts and blocked brute force protection.

This change raises the log level from CUPSD_LOG_DEBUG to
CUPSD_LOG_WARN for the "User not in group(s)" message in
scheduler/auth.c, ensuring it appears in logs regardless of
the configured LogLevel.

[abubakarsabir924-cell]

Hi @michaelrsweet Sir, I have submitted PR #[PR 1561] for this issue. I changed the log level from CUPSD_LOG_DEBUG to CUPSD_LOG_WARN in scheduler/auth.c line 2084, where the 'User not in group(s)' message is logged before returning HTTP_STATUS_UNAUTHORIZED. This ensures authentication failures are visible when LogLevel is set to error, enabling fail2ban to work correctly with CUPS. Please review and let me know if any changes are needed.