Improve story for outdated distros content

[tfoote]

The Kinetic indexing is taking a long time and is broken because gitlab.uni-koblenz.de has gone offline taking several release repositories offline such as:

https://github.com/ros/rosdistro/blob/ed910093813662356c8b49736423bb78f1fbe3c4/kinetic/distribution.yaml#L4998

The code is correctly timing out and continuing, however the timeout is ~2 minutes and is blocking the indexing process. And there's dozens of these.

At some level we shouldn't expect old/outdated content to stay available/valid forever. Though with the ros2-gbp hosting it will be more reliable.

There's a level that we should probably turn off indexing of older distros. Though it's nice to have the older data available for historical purposes. It should probably be moved to a snapshot archive instead of reindexing it every time.

[rkent]

I said in #337 that the stats problem would be solved by reducing the distro count. I believe we should eliminate all distros older than melodic and galactic from rosindex.