Replace Clock().now() to node.get_clock().now() in diagnostic_updater.py

[h-wata]

Description

In this PR, the Clock import in the diagnostic_updater.py has been replaced with node.get_clock().
This ensures that the correct ROS_TIME is used when the use_sim_time parameter is set in the ros2 node.

Environment

ros 2 : Humble

Ref

• https://answers.ros.org/question/321536/replacement-for-rospytimenow-in-ros2/
• https://design.ros2.org/articles/clock_and_time.html

Verifying the Changes

• Terminal1

ros2 launch gazebo_ros gazebo.launch.py

• Terminal2

ros2 run diagnostic_updater example.py --ros-args -p use_sim_time:=true # or false

• Terminal3

ros2 topic echo /diagnostics --field header.stamp.sec

Before

header.stamp is System Time regardless of use_sim_time.

After

header.stamp is ROS Time when use_sim_time is true.

[ct2034]

Thanks for your pull request 👍