Stop using caribou

Clutter and related libraries were [archived](discourse.gnome.org/t/retiring-clutter-and-friends/8949) by their maintainers in 2022. Caribou was archived in 2023 (although I tried to archive it sooner in 2019 but it looks like I didn't see a follow-up request). Caribou is now the last thing in Debian using Clutter and Caribou is only used by cinnamon and cinnamon-screensaver. It appears that cinnamon recently stopped [using Caribou](https://github.com/linuxmint/cinnamon/commit/5fd5748988a1) [although a new upstream release is still needed], but cinnamon-screensaver is still using Caribou. By the way, there was earlier discussion in the lengthy #354 about stopping using caribou.