image_pipeline: update branch for iron (#39567)

Author: mikeferguson

iron/distribution.yaml

@@ -2216,7 +2216,7 @@ repositories:
     doc:
       type: git
       url: https://github.com/ros-perception/image_pipeline.git
-      version: rolling
+      version: iron
     release:
       packages:
       - camera_calibration
@@ -2236,7 +2236,7 @@ repositories:
       test_pull_requests: true
       type: git
       url: https://github.com/ros-perception/image_pipeline.git
-      version: rolling
+      version: iron
     status: maintained
   image_transport_plugins:
     doc: