chore(clustering): adjust error log levels for control plane connections (#13863)

Changed error log level from ERROR to WARN for better clarity and to avoid
alarming users unnecessarily when control plane connections break.
This adjustment enhances user experience under connection interruptions.

Fix:[FTI-6238](https://konghq.atlassian.net/browse/FTI-6238)

Signed-off-by: tzssangglass <[email protected]>
(cherry picked from commit af9eb5e)

Author: tzssangglass

changelog/unreleased/kong/chore-clustering-log-level.yml

@@ -0,0 +1,3 @@
+message: "**Clustering**: Adjust error log levels for control plane connections."
+type: bugfix
+scope: Clustering

kong/clustering/data_plane.lua

@@ -164,7 +164,7 @@ function _M:communicate(premature)
 
   local c, uri, err = clustering_utils.connect_cp(self, "/v1/outlet")
   if not c then
-    ngx_log(ngx_ERR, _log_prefix, "connection to control plane ", uri, " broken: ", err,
+    ngx_log(ngx_WARN, _log_prefix, "connection to control plane ", uri, " broken: ", err,
                  " (retrying after ", reconnection_delay, " seconds)", log_suffix)
 
     -- load config from backup storage (DP-Resilience)