Handle removal of legacy access restriction

[petersutter]

What this PR does / why we need it:
With gardener/gardener#10883, Gardener no longer automatically removes legacy fields when .spec.accessRestrictions is deleted. This behavior specifically applies to the eu-access-only access restriction. To fully disable the eu-access-only restriction for a Shoot, the legacy fields must also be explicitly removed. This PR implements the necessary changes to handle this scenario.

Additionally, this PR updates the access restriction configuration of the dashboard at runtime by mapping accessRestriction.items[@key="seed.gardener.cloud/eu-access"] to accessRestriction.items[@key="eu-access-only"]. This mapping will be removed in a future version of the dashboard and operators are expected to adapt their config accordingly, if applicable.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

**Action Required:** The automatic mapping from `accessRestriction.items[@key="seed.gardener.cloud/eu-access"]` to `accessRestriction.items[@key="eu-access-only"]` will be removed in a future dashboard version. Dashboard operators who use `seed.gardener.cloud/eu-access` in their access restriction configuration should update the key to `eu-access-only`. If your configuration does not include `seed.gardener.cloud/eu-access`, no action is required.