CI: Run lock.yml only once a week

[why]
After the initial locking of many issues (where we called the workflow
once every hour, it can only process 50 items in one run), this high
frequency is not needed anymore.

[how]
Just search for old closed issues once a week on monday morning.
Also remove manual trigger.

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

.github/workflows/lock.yml

@@ -2,8 +2,7 @@ name: 'Lock Threads'
 
 on:
   schedule:
-    - cron: '0 * * * *'
-  workflow_dispatch:
+    - cron: '0 0 * * 1'
 
 permissions:
   issues: write