Mark Melodic as End-of-Life.

[clalancette]

Note to rosdistro reviewers to please not merge this right away; I'll do it once we are ready.

@nuclearsandwich Copying and modifying your list from #37746 (comment):

Melodic build farm wind-down

• Stop merging all melodic-related ros/rosdistro PRs
• Disable melodic jobs from ros-infrastructure/ros_buildfarm_config: Completely disable Melodic jobs. ros-infrastructure/ros_buildfarm_config#223
• Disable jobs using the method described in https://github.com/ros-infrastructure/ros_buildfarm/blob/master/doc/ongoing_operations.rst (but with batching)
• Merge this PR
• Update distro in https://github.com/ros-infrastructure/rosindex/blob/ros2/_config.yml
• After a few weeks, assuming that no one has asked to resurrect melodic for a fixup sync, the jobs can be deleted using a modified copy of the disabling script.

[mikaelarguedas]

+ create a final snapshot made available on snapshots.ros.org 🙏 (cross-referencing osrf/docker_images#660 (comment)))
It's been done 👍