Use html2pango::block to render html
Deprecates !11 (closed). It also gives more control over how the parsing is being done, for an example see https://gitlab.gnome.org/GNOME/fractal/-/merge_requests/399.
This can be merged as is with only one drawback, paragraphs are concatenated instead of being separated by newline.
A MR which is way simpler than the previous one was filed at World/html2pango!12 (merged). This one should be easier to merge.