Remove iron workflows and update readme (#1338)

Co-authored-by: Dr. Denis <[email protected]>

.github/dependabot.yml

@@ -18,10 +18,3 @@ updates:
     schedule:
       interval: "weekly"
     target-branch: "humble"
-  - package-ecosystem: "github-actions"
-    # Workflow files stored in the
-    # default location of `.github/workflows`
-    directory: "/"
-    schedule:
-      interval: "weekly"
-    target-branch: "iron"

.github/mergify.yml

@@ -8,15 +8,6 @@ pull_request_rules:
         branches:
           - humble
 
-  - name: Backport to iron at reviewers discretion
-    conditions:
-      - base=master
-      - "label=backport-iron"
-    actions:
-      backport:
-        branches:
-          - iron
-
   - name: Ask to resolve conflict
     conditions:
       - conflict

README.md

@@ -11,7 +11,6 @@ ROS2 Distro | Branch | Build status | Documentation | Released packages
 :---------: | :----: | :----------: | :-----------: | :---------------:
 **Rolling** | [`master`](https://github.com/ros-controls/ros2_controllers/tree/master) | [![Rolling Binary Build](https://github.com/ros-controls/ros2_controllers/actions/workflows/rolling-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_controllers/actions/workflows/rolling-binary-build.yml?branch=master) <br /> [![Rolling Semi-Binary Build](https://github.com/ros-controls/ros2_controllers/actions/workflows/rolling-semi-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_controllers/actions/workflows/rolling-semi-binary-build.yml?branch=master) | [control.ros.org](https://control.ros.org/master/doc/ros2_controllers/doc/controllers_index.html) | [ros2_controllers](https://index.ros.org/p/ros2_controllers/#rolling)
 **Jazzy** | [`master`](https://github.com/ros-controls/ros2_controllers/tree/master) | [![Jazzy Binary Build](https://github.com/ros-controls/ros2_controllers/actions/workflows/jazzy-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_controllers/actions/workflows/jazzy-binary-build.yml?branch=master) <br /> [![Jazzy Semi-Binary Build](https://github.com/ros-controls/ros2_controllers/actions/workflows/jazzy-semi-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_controllers/actions/workflows/jazzy-semi-binary-build.yml?branch=master) | [control.ros.org](https://control.ros.org/jazzy/doc/ros2_controllers/doc/controllers_index.html) | [ros2_controllers](https://index.ros.org/p/ros2_controllers/#jazzy)
-**Iron** | [`iron`](https://github.com/ros-controls/ros2_controllers/tree/iron) | [![Iron Binary Build](https://github.com/ros-controls/ros2_controllers/actions/workflows/iron-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_controllers/actions/workflows/iron-binary-build.yml?branch=master) <br /> [![Iron Semi-Binary Build](https://github.com/ros-controls/ros2_controllers/actions/workflows/iron-semi-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_controllers/actions/workflows/iron-semi-binary-build.yml?branch=master) | [control.ros.org](https://control.ros.org/iron/doc/ros2_controllers/doc/controllers_index.html) | [ros2_controllers](https://index.ros.org/p/ros2_controllers/#iron)
 **Humble** | [`humble`](https://github.com/ros-controls/ros2_controllers/tree/humble) | [![Humble Binary Build](https://github.com/ros-controls/ros2_controllers/actions/workflows/humble-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_controllers/actions/workflows/humble-binary-build.yml?branch=master) <br /> [![Humble Semi-Binary Build](https://github.com/ros-controls/ros2_controllers/actions/workflows/humble-semi-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_controllers/actions/workflows/humble-semi-binary-build.yml?branch=master) | [control.ros.org](https://control.ros.org/humble/doc/ros2_controllers/doc/controllers_index.html) | [ros2_controllers](https://index.ros.org/p/ros2_controllers/#humble)
 
 ### Explanation of different build types