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.

---------

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

.github/workflows/cla.yml

@@ -46,7 +46,7 @@ jobs:
     # Echoes one line; the run exists only so the concurrency group cancels the
     # in-flight run of a closed PR.
     permissions: {}
-    runs-on: ubuntu-latest
+    runs-on: sm-standard-2
     timeout-minutes: 10
     steps:
       - name: Explain cancellation run
@@ -61,7 +61,7 @@ jobs:
       checks: write
       issues: write
       pull-requests: write
-    runs-on: ubuntu-latest
+    runs-on: sm-standard-2
     timeout-minutes: 30
     steps:
       - name: Report CLA result for merge queue