ikiwiki:trixie container image: update po4a and ikiwiki

Context: https://gitlab.tails.boum.org/tails/tails/-/issues/20581

• install po4a from Trixie
• install ikiwiki 3.20260201 or newer (e.g. from Forky, or from sid, or from our custom APT repo, whatever sysadmins prefer)

As requested on February 2, it would be good if this was completed by the end of the month. Does this sound realistic?