Milestone for librsvg 2.54.0, to coincide with the GNOME 42 release.

Things I'd like to have:

• Reference documentation migrated to gi-docgen. done, #827 (closed)
• rsvg-convert man page done with Sphinx. done, rst2man instead, #755 (closed)
• Update gtk-rs. done, !634 (merged)
• Updated CI image for new Pango stack, with updated test reference files. done, !654 (merged)
• Cargo-c instead of libtool???
• Meson instead of autotools???

GNOME 42 tarballs due:

• alpha - Saturday 2022/Jan/08
• beta - Saturday 2022/Feb/12 (feature freeze, API/ABI freeze)
• release candidate - Saturday 2022/Mar/05 (hard code freeze)
• GNOME 42.0 tarballs - Saturday 2022/Mar/19