Drop support for the shoot.gardener.cloud/use-as-seed annotation

[ialidzhikov]

How to categorize this PR?
/area quality
/kind cleanup
/kind api-change

What this PR does / why we need it:
This PR drops the support for the shoot.gardener.cloud/use-as-seed annotation by removing the gardenlet's shooted seed registration (SSR) controller.

Which issue(s) this PR fixes:
Part of #6305

Release note:

The already deprecated `shoot.gardener.cloud/use-as-seed` annotation (since v1.18.0) is no longer supported for creating Shooted Seed clusters. Please check the following [documentation](https://github.com/gardener/gardener/blob/v1.51.0/docs/usage/managed_seed.md#migrating-from-the-use-as-seed-annotation-to-managedseeds) on how to migrate from the `use-as-seed` annotation to `ManagedSeeds`. Before updating to this version of Gardener, make sure that you migrated to `ManagedSeeds` and that you no longer have usages of the `use-as-seed` annotation on the landscape.

[gardener-prow]

@ialidzhikov: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gardener-apidiff	0be11b0	link	false	/test pull-gardener-apidiff

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[timebertt]

/assign

[timebertt]

Very nice PR, thank you!

I just found one left-over from the migration that is actually not directly related to this PR.
The PR as such
/lgtm

[timebertt]

I just figured out, that there is no similar validation for Shoots referenced by ManagedSeeds.

There is validation that denies enabling the ingress controller on ManagedSeeds if the referenced Shoot already enables the addon:

gardener/plugin/pkg/managedseed/validator/admission.go

Lines 282 to 285 in 2e3d605

	if spec.Ingress != nil {
	if gardencorehelper.NginxIngressEnabled(shoot.Spec.Addons) {
	allErrs = append(allErrs, field.Forbidden(fldPath.Child("ingress"), fmt.Sprintf("seed ingress controller cannot be enabled on shoot %s", kutil.ObjectName(shoot))))
	}

However, there is no validation for the other direction. I.e., you can update the Shoot to enable the nginx ingress addon, even if the referencing ManagedSeed already enables the addon.

This is only partly related to this PR, but I'm wondering whether we such add such validation.
(Maybe it's not worth the effort though and rather an edge case that doesn't need validation)
Looking for opinions :)

[ialidzhikov]

However, there is no validation for the other direction. I.e., you can update the Shoot to enable the nginx ingress addon, even if the referencing ManagedSeed already enables the addon.

Valid point, it seems that I overlooked this case. For now I can try to preserve the existing logic. Instead of checking for the shootUseAsSeed annotation, I can try to check if the Shoot is ManagedSeed, and if yes -> forbid enabling the nginx-ingress addon from the Shoot spec.

[ialidzhikov]

I decided to add this validation as part of the PR that adds the VPA "enforcement" - ref #6388 (WIP for now as I have to test thoroughly and add docs about the new validations that we introduce).

PS. Now going one more time over the discussion, I guess I also need to do the changes in ShootValidation admission plugin because the ManagedSeed admission plugin won't be invoked on changes to the Shoot spec.
TL;DR let's do this as part of #6388. As you shared in today's master we don't have such validation for ManagedSeeds (only for the legacy use-as-seed annotation).

[timebertt]

Thank you for taking care! Sounds good to me :)

[timebertt]

/approve

[gardener-prow]

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This pull-request has been approved by: timebertt

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