OS update recommendation should take CRI support into account

[voelzmo]

What would you like to be added:
As a user, I'd like to get recommendations for updating my OS only if it supports the currently selected container runtime for my Shoot.

Why is this needed:
In the future, we might release e.g. a new gardenlinux version without support for docker. Shoot owners who have docker configured for their worker should not get the recommendation to update to this version.

[grolu]

@voelzmo Please have a look at the referenced PR. We still recommend the update, I think it is not a solution tom stay on an outdated OS anyway. But we make the user aware that changing the OS will require a change of the Container Runtime. This change is no longer made implicitly. The user needs to explicitly select a runtime so he cannot accidentally change the container runtime by upgrading the worker image. I hope this is sufficient.

[voelzmo]

Only seeing this now, sorry. Looks good to me, especially given that going forward people will have to pick containerd anyways for newer k8s versions and all of our OS images now support that.