Remove IUKs older than 6 months old from /{stable,alpha}/iuk

As of now, the size of those mirrors is ever growing as
we are keeping all past IUK:

We could limit that amount of data by moving the ones that are more than 6 months old to /tails/obsolete/iuk/.

Let’s do try to do that manuall for a few releases.

The first step is to document how to do this in the release process.

Original created by @sajolida on 8479 (Redmine)

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