Switch to a Bookworm (from Buster) image for gitlab-ci

• Buster LTS will be EOL in 3 months.
• Switch to python3-lxml since the python2 version is no longer available with Bookworm.
• Convert hotdoc-html-2-pdf to python3
• Adjust dependencies for Bookworm

This MR includes only minimal changes to make it compatible with Bookworm, but since there were a lot of changes between python2 vs python3 and texlive 2018 vs texlive 2022, I guess we can maybe optimize some parts. But at least, we are no longer stuck with Buster here.

I compared some of the generated pdf and they look good (and similar).