switch GCR -> Artifact-Registry

[ccwienk]

GCR has been deprecated [0] in favour of Artifact-Registry.

Thus, change push-targets for OCI-Images:

• europe-docker.pkg.dev/gardener-project/snapshots for snapshots
• europe-docker.pkg.dev/gardener-project/releases for releases
• europe-docker.pkg.dev/gardener-project/public for combined view of snapshots + releases

[0] https://cloud.google.com/artifact-registry/docs/transition/transition-from-gcr

Release note:

Change OCI Image Registry from GCR (`eu.gcr.io/gardener-project`) to Artifact-Registry (`europe-docker.pkg.dev/gardener-project/releases`). Users should update their references.

[gardener-robot-ci-2]

This PR proposes changes that would break the pipeline definition:

external-dns-management-gcr-to-ar: Traceback (most recent call last):
  File "/usr/lib/python3.11/site-packages/concourse/replicator.py", line 141, in render
    definition_descriptor = self._render(definition_descriptor)
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/concourse/replicator.py", line 185, in _render
    'definition': factory.create_pipeline_definition(),
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/concourse/factory.py", line 108, in create_pipeline_definition
    validator.validate()
  File "/usr/lib/python3.11/site-packages/concourse/validator.py", line 29, in validate
    self._validate_variant(variant)
  File "/usr/lib/python3.11/site-packages/concourse/validator.py", line 34, in _validate_variant
    self._validate_element(variant)
  File "/usr/lib/python3.11/site-packages/concourse/validator.py", line 38, in _validate_element
    self._validate_element(child, parents | {element})
  File "/usr/lib/python3.11/site-packages/concourse/validator.py", line 39, in _validate_element
    element.validate()
  File "/usr/lib/python3.11/site-packages/concourse/model/traits/publish.py", line 397, in validate
    super().validate()
  File "/usr/lib/python3.11/site-packages/model/base.py", line 44, in validate
    self._validate_known_attributes()
  File "/usr/lib/python3.11/site-packages/model/base.py", line 73, in _validate_known_attributes
    raise ModelValidationError(
model.base.ModelValidationError: PublishTrait:publish: The following attributes are unknown: dns-controller-manager.

[gardener-robot-ci-1]

The pipeline-definition has been fixed.