odcds: log abnormal message in higher level (envoyproxy#37529)

The reason that client request can't find the on-demand cluster can be:
1. the on-demand request is not triggered
2. the requested cluster is not found
3. the request timed out

In the previous situation, the log level of failure reason 2/3 is
'debug', which is too low that we can't distinguish the reason via log.
This change increases the log level so that user can troubleshoot the
issue directly.

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Commit Message: odcds: log abnormal message in higher level
Additional Description:
Risk Level: Low, only log level changed
Testing:
Docs Changes:
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional [API
Considerations](https://github.com/envoyproxy/envoy/blob/main/api/review_checklist.md):]

Signed-off-by: spacewander <[email protected]>

source/common/upstream/cluster_manager_impl.cc

@@ -1781,12 +1781,12 @@ ClusterManagerImpl::requestOnDemandClusterDiscovery(OdCdsApiSharedPtr odcds, std
 }
 
 void ClusterManagerImpl::notifyMissingCluster(absl::string_view name) {
-  ENVOY_LOG(debug, "cm odcds: cluster {} not found during on-demand discovery", name);
+  ENVOY_LOG(info, "cm odcds: cluster {} not found during on-demand discovery", name);
   notifyClusterDiscoveryStatus(name, ClusterDiscoveryStatus::Missing);
 }
 
 void ClusterManagerImpl::notifyExpiredDiscovery(absl::string_view name) {
-  ENVOY_LOG(debug, "cm odcds: on-demand discovery for cluster {} timed out", name);
+  ENVOY_LOG(info, "cm odcds: on-demand discovery for cluster {} timed out", name);
   notifyClusterDiscoveryStatus(name, ClusterDiscoveryStatus::Timeout);
 }