Update cursive to 0.21.0

[gyscos]

Describe your changes

This updates cursive to 0.21.0. This release includes a buffered backend internally, so there is no need for a separate cursive_buffered_backend crate.

The main change is that View now require Send + Sync, so generic views should forward these requirements.
Note that the actual views here were already Send + Sync, and no logic change was needed.

Issue ticket number and link

Checklist before requesting a review

• Documentation was updated (i.e. due to changes in keybindings, commands, etc.)
• Changelog was updated with relevant user-facing changes (eg. not dependency updates,
  not performance improvements, etc.)

[gyscos]

I think the CI failure is caused by the new ncurses crate requiring a proper TERM variable to be set (for example to xterm) during compilation, since it relies on ncurses itself in the build script to "discover" some constants.

Workarounds include injecting TERM=xterm in the CI process, or dropping the ncurses backend entirely.
I'm adding a commit to this PR to do the former, but it could be dropped it the latter is preferred instead.

[hrkfdn]

Thanks for the PR ❤️, LGTM! The CD workflows probably also need TERMto be adjusted, but we/I can figure this out later.