[infra] Give up the old distfiles path/mirror

Now that `mark-devkit` can generate both golang and rust packages we can schedule the remove of the old distfiles path in hash mode.

It's fine keep only the flat distfiles updated fron mark-devkit.

These the tasks in order to close this issue:

* [ ] run `mark-devkit kit distfiles-sync` in every branch and supported tags and fix broken links
* [x] bump a new sys-apps/portage revision in order to change GENTOO_MIRRORS default value (write a note in documentation about how override this variable for old rootfs)
* [ ] add `mark-devkit kit distfiles-clean` command to purge tarballs no more used

I think that we can drop the old path in September/October 2025.

Start with the mark-devkit entry points for `kit distfiles-sync` and the proposed `kit distfiles-clean` command, then inspect the supported branches and tags listed in the issue. Done means synchronization has no broken links across supported branches and tags, and the clean command purges tarballs that are no longer used so the old path can be removed.

Written by the indexing model from the issue text.