[CI] run renovate daily instead of every 30 minutes

It was necessary in the debug period to help with a faster debug
loop. Now that it works reliably, there is no need for renovate
updates more than once a day.

It will still possible to force a run, should it be necessary, by
re-running the last scheduled job.

.forgejo/workflows/renovate.yml

@@ -1,3 +1,11 @@
+#
+# The 2am run will rebase what needs rebasing & trigger the CI
+# The 4am run will merge one of them
+#
+# These times are chosen to minimize the likelyhood that another PR
+# is merged at the same time. This would not be necessary if automerge
+# worked but as of 30 March 2024 it does not.
+#
 name: renovate
 
 on:
@@ -5,7 +13,7 @@ on:
     branches:
       - 'renovate/**' # self-test updates
   schedule:
-    - cron: '*/30 * * * *'
+    - cron: '0 2,4 * * *'
 
 env:
   RENOVATE_DRY_RUN: ${{ (github.event_name != 'schedule' && github.ref_name != github.event.repository.default_branch) && 'full' || '' }}