Follow ROS security updates & remove EOL distro Indigo

[MichaelGrupp]

GPG keys of pakages.ros.org have changed because of a security issue:
https://discourse.ros.org/t/security-issue-on-ros-build-farm/9342
https://discourse.ros.org/t/new-gpg-keys-deployed-for-packages-ros-org/9454

The melodic base image of cartographer_ros has the outdated key. This leads to problems while updating rosdeps (if no caching is used), see this CI log of our dummy repository:

Err:2 http://packages.ros.org/ros/ubuntu bionic InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654

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Additionally, ROS Indigo has reached EOL and can be removed from CI builds. They are currently broken because of that.

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• re-build & push base image (pulls new image with fresh keys)
• remove Indigo CI (done in Remove EOL Distros from CI matrix #1379)

[bochen87]

@MichaelGrupp I just noticed CI is broken since merge of #1379
Do you know who has access to the melodic base image? It seems to be stored on the Google Cloud Platform

[bochen87]

should be fixed by #1484