capd: fix kindest/node images

Related to #1743 (closed)

This MR forces the use of an 1.29.8 image for kubeadm-capd (there isn't one available for 1.29.9 yet).

For RKE2, need sylva-projects/sylva-elements/container-images/rke2-in-docker!29 (merged) so that the image is actually built.