Replace docbook2odf or remove functionality
docbook2odf
is an optional package used by gimp-help to convert the help to OpenDocument files.
However, this package was removed from Debian in 2019. It was unmaintained.
Last release was apparently from 2007, and there was some development until 2013.
If someone knows another package that we can use instead, your input is welcome. Otherwise, I suggest dropping support for ODF.