adjust woodpecker.yml syntax to next version

.woodpecker.yml

@@ -1,4 +1,7 @@
-pipeline:
+when:
+  event: tag
+
+steps:
   build-push:
     image: plugins/docker
     settings:
@@ -13,5 +16,3 @@ pipeline:
         from_secret: docker_username
       password:
         from_secret: docker_password
-    when:
-      event: tag