They reappear in the newest adaptive layout wireframes.
These were dropped in
54a8445abb ("window: Drop the back and forward buttons")
Closes: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/933
Signed-off-by: Markus Göllnitz <camelcasenick@bewares.it>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/542>