Rm ocm retention policy

[zkdev]

Google Artifact Registry has a built-in mechanism.

Release note:

OCM clean-up logic (based on version-policies) is removed (we use oci-registry built-ins instead).

[ccwienk]

removing this seems like a good idea. however, there are quite a few pipelines that configured this explicitly. I suggest to remove existing configurations prior to dropping this codepath

[zkdev]

@ccwienk
I kept the attribute spec exactly due to this, so we can clean up afterwards.

[ccwienk]

@zkdev : I understood that. Still, I do not think we should just silently change pipeline-contract w/o rolling this out explicitly (there might be pipelines that actually rely on this cleanup-code).

[gardener-robot]

@TuanAnh17N, @8R0WNI3, @AndreasBurger You have pull request review open invite, please check