Weblate staging website: update po4a and ikiwiki

Just like the container image (#18262) except we need explicit pinning, not just "right now we are on Trixie so we'll get the right version of po4a by chance", which would break stuff once we upgrade the VM to Forky.

The code lives in profile::tails::website::builder.

Priority: I have no idea, since I have no visibility about how much it's used, and I understand there are other blockers anyway, so I'll let you judge what timing is appropriate.