cloud_storage_clients/client_pool: error->warn

this message does not need to be at error level. in case this function
fails to produce a result, the caller will print an ERROR message anyway

(cherry picked from commit 2209621)

src/v/cloud_storage_clients/client_pool.cc

@@ -121,7 +121,7 @@ client_pool::do_client_self_configure(http_client_ptr client) {
 
             if (result.error() == cloud_storage_clients::error_outcome::retry) {
                 vlog(
-                  pool_log.error,
+                  pool_log.warn,
                   "Self configuration attempt {}/{} failed with retryable "
                   "error. "
                   "Will retry in {}s.",