chore(ci): run contributors workflow monthly instead of weekly (#281)

The all-time contributors workflow no longer needs a weekly cadence:
nowadays it mostly bumps maintainer activity. Run it on the 1st of
each month at 04:49 UTC ('49 4 1 * *') instead of every Saturday.

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: fredbi

.github/workflows/contributors.yml

@@ -2,7 +2,7 @@ name: Contributors
 
 on:
   schedule:
-    - cron: '18 4 * * 6'
+    - cron: '49 4 1 * *'
 
   workflow_dispatch: