Retire images based on xenial, buster and bionic due to reaching EOL

[tfoote]

Xenial has reached EOL and we should turn off our builds for them.

We were pinged: docker-library/official-images#13950 (comment)

Soon we will also EOL Bionic too for ros and gazebo builds

docker-library/official-images#13950 (comment)

[mikaelarguedas]

I was waiting for Gazebo 9 to reach EOL, as it was announced yesterday so I retired the gazebo 9 images.

Future work:
End of May:

• Retire gazebo 11 bionic image Retire gazebo bionic #679
• Switch melodic to snapshots apt repo then retire builds

[mikaelarguedas]

Last action item of this issue is retiring melodic builds. This is blocked pending melodic final sync to hit the snapshots repository.
Not sure we will be able to build the final image as the bionic base image was retired: docker-library/official-images#14867 (comment) docker-library/official-images#14934 (comment)