Update pendulum-example

[christophfroehlich]

For demonstration purposes, I change the initial pose to be in the lower equilibrium to make this clearer: https://github.com/ros-controls/gazebo_ros2_control/assets/3367244/630cc8b8-6cb4-4dbc-9caa-948c085052e0

Originally posted by @christophfroehlich in https://github.com/ros-controls/gazebo_ros2_control/issues/172#issuecomment-2105185043

and make position and effort variants equal, as requested here https://github.com/ros-controls/gazebo_ros2_control/pull/172#pullrequestreview-2024406743

[christophfroehlich]

@mergifyio backport humble iron

[mergify[bot]]

backport humble iron

✅ Backports have been created

• #303 Update pendulum-example (backport #301) has been created for branch humble
• #304 Update pendulum-example (backport #301) has been created for branch iron