Reduce stale bot to run once per week

I have the impression that we are not systematically reviewing the
issues marked as inactive. We had multiple cases of re-opening issues,
which means they haven't been identified as "keep" during the
stale phase. IMHO this should not happen as it increases the danger of
overlooking the
 closing of relevant issues.

Author: timhoffm

.github/workflows/stale.yml

@@ -2,7 +2,7 @@
 name: 'Label inactive PRs'
 on:
   schedule:
-    - cron: '30 1 * * 1,3,5'
+    - cron: '30 1 * * 1'
 
 jobs:
   stale: