Reintroduce seed zones admission checks

[timuthy]

How to categorize this PR?

/area high-availability
/kind api-change
/kind technical-debt

What this PR does / why we need it:
This PR reintroduces admission checks for seed zones, removed by 8d28452. In the meantime, the Azure provider extension can deal with a zone mismatch on its own, see gardener/gardener-extension-provider-azure#602.

Which issue(s) this PR fixes:
Part of #6529

Special notes for your reviewer:
/cc @kon-angelo

Release note:

`Seed` and `ManagedSeed` API validation has been enhanced by the following checks:
(a) `ManagedSeed`s can only use the very same zone(s) (`managedSeed.spec.gardenlet.config.seedConfig.spec.provider.zones`) that are available in the registered `Shoot` (`shoot.spec.provider.workers[].zones`).
(b) `seed.spec.provider.zones` can only be removed if no shoots are scheduled to affected seed.
These restrictions were removed in Gardener `v1.60` to compensate a zone mismatch issues in Azure that is in the meantime fixed by the Azure provider extension [v1.33](https://github.com/gardener/gardener-extension-provider-azure/releases/tag/v1.33.0).
⚠️ Before upgrading to this Gardener version, please make sure the zone configuration of your seeds matches the ones from registered shoots, i.e. `seed.spec.provider.zones == shoot.spec.provider.workers[].zones`.

[rfranzke]

/lgtm
/assign @kon-angelo

[gardener-prow]

LGTM label has been added.

Git tree hash: b837c5e77afe6d9801cd903138d8312a16e5948b

[kon-angelo]

/lgtm

In the removal PR there were some additional tests for the miscellaneous.go file that can probably be reused.
You can notify me for azure headaches/bugs but it should work now.

[rfranzke]

@kon-angelo I've added the missing tests from the linked commit again with bf8edc2.

/lgtm
/approve

[gardener-prow]

LGTM label has been added.

Git tree hash: c22df0db59b6d49cf50636620977f3510675ff8d

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rfranzke

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[gardener-prow]

@timuthy: The following tests 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-e2e-kind-ha-single-zone-upgrade	bf8edc2	link	false	/test pull-gardener-e2e-kind-ha-single-zone-upgrade
pull-gardener-e2e-kind-ha-multi-zone-upgrade	bf8edc2	link	false	/test pull-gardener-e2e-kind-ha-multi-zone-upgrade

Full PR test history. Your PR dashboard. Command help for this repository.
Please help us cut down on flakes by linking this test failure to an open flake report or filing a new flake report if you can't find an existing one. Also see our testing guideline for how to avoid and hunt flakes.

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[rfranzke]

/test pull-gardener-integration