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/minio-interop.yml

@@ -62,7 +62,7 @@ jobs:
     name: MinIO interop (EC + SSE read parity)
     # Skip on forks: needs the repo's runners and is not a contributor gate.
     if: github.repository == 'rustfs/rustfs'
-    runs-on: ubuntu-latest
+    runs-on: sm-standard-2
     timeout-minutes: 40
     env:
       # Fixed 32-byte test KMS key baked into the fixture lab; not a secret.
@@ -131,7 +131,7 @@ jobs:
     name: Alert on scheduled failure
     needs: [minio-interop]
     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