feat(core): log at ERROR level for script logs to stderr

Fixes kestra-io/plugin-scripts#190

Author: loicmathieu

Diff for: core/src/main/java/io/kestra/core/models/tasks/runners/PluginUtilsService.java

@@ -201,7 +201,7 @@ public static Map<String, Object> parseOut(String line, Logger logger, RunContex
             }
         } else {
             if (isStdErr) {
-                runContext.logger().warn(line);
+                runContext.logger().error(line);
             } else {
                 runContext.logger().info(line);
             }