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

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 <tzssangglass@gmail.com>

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

@@ -153,7 +153,7 @@ 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)
 
     assert(ngx.timer.at(reconnection_delay, function(premature)