capo/capm3: don't enable ubuntu-jammy-hardened-rke2 image by default

(applies to capo and capm3)

Today we enable by default the ubuntu-jammy-hardened-rke2 OS image (for latest Kubernetes).

The benefit of this is only for a (hypothetical) fraction of Sylva users (using capo or capm3 with Ubuntu and RKE2, with the hardened flavor): they won't need to explicitly enable this image.

And this comes with a drawback:

• in CI we have to remember disabling this image each time it changes (e.g. sylva-projects/sylva-elements/ci-tooling/ci-deployment-values!138 (merged)) or else our capo runs will download it additionally to the one relevant for the run
• real users, or people building Sylva derivatives, and who want to use a different image, will also want to disable it, and will also have to notice the change or they'll download it by default

This MR is a proposal to stop enabling this image by default.

(Goes together with sylva-projects/sylva-elements/ci-tooling/ci-deployment-values!139 (merged))