Release procedure: drop old releases from dist.apache.org

[eolivelli]

We have to upgrade the release guide in order to drop old releases from dist.apache.org.
We should keep only 1 version per active branch

[eolivelli]

Marking this as "important" because we have to fix this at least at next major release (when we will create a new release branch)

[sijie]

@eolivelli : there is already a section "Cleanup old releases" in the release guide, no?

[eolivelli]

I missed that. Sorry