ci: replace ubuntu-latest runner with sm-standard-2 across workflows (#8112)

* ci: replace ubuntu-latest runner with sm-standard-2 across workflows

* ci: keep scheduled-validation monitors on hosted runners

The freshness and watchdog jobs report stalled scheduled validations.
Running them on the same sm-standard-2 pool means a pool outage stalls
the monitors too, so nothing reports it.

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Co-authored-by: overtrue <anzhengchao@gmail.com>

.github/workflows/fuzz.yml

@@ -60,7 +60,7 @@ jobs:
   cancel-closed-pr-runs:
     name: Cancel Closed PR Runs
     if: github.event_name == 'pull_request' && github.event.action == 'closed'
-    runs-on: ubuntu-latest
+    runs-on: sm-standard-2
     timeout-minutes: 10
     steps:
       - name: Explain cancellation run
@@ -240,7 +240,7 @@ jobs:
     # job fails. Alerts only for scheduled (nightly) runs (backlog#1149
     # ci-8); PR and manual dispatch failures are already watched by a human.
     if: always() && github.event_name == 'schedule' && contains(needs.*.result, 'failure')
-    runs-on: ubuntu-latest
+    runs-on: sm-standard-2
     timeout-minutes: 10
     permissions:
       contents: read