Move config dict combination to python layer, remove 21.9 support

I'd like to start reading in the configuration in the python when I can, so that we can do things like use a consistent jet collection name for component accumulator code, or use the json to toggle various CA-based config. This is a start in that direction.

It needed configuration file merging, which I moved to Athena in atlas/athena!67027 (merged).

While I was working on this I had some issues with the 21.9 upgrade tests. @tstreble tells me that we don't need this any more, so I'm completely removing support for that as well. This is thus a pretty big deletion as well.

I'm explicitly not squashing this to factorize the upgrade code removal from the release bump / configuration changes.