GCR -> Artifact-Registry (#296)

* cleanup: rm obsolete/redundant attributes

* switch GCR -> Artifact-Registry

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 read-access to
  snapshots + releases

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

.ci/pipeline_definitions

@@ -1,5 +1,4 @@
 autoscaler:
-  template: 'default'
   base_definition:
     repo:
       source_labels:
@@ -19,6 +18,8 @@ autoscaler:
         preprocess:
           'inject-commit-hash'
         inject_effective_version: true
+      component_descriptor:
+        ocm_repository: europe-docker.pkg.dev/gardener-project/snapshots
       publish:
         oci-builder: docker-buildx
         platforms:
@@ -31,8 +32,7 @@ autoscaler:
                 source: ~ # default
               steps:
                 build: ~
-            registry: 'gcr-readwrite'
-            image: 'eu.gcr.io/gardener-project/gardener/autoscaler/cluster-autoscaler'
+            image: europe-docker.pkg.dev/gardener-project/snapshots/gardener/autoscaler/cluster-autoscaler
             dockerfile: './cluster-autoscaler/Dockerfile'
     steps:
       test:
@@ -44,7 +44,8 @@ autoscaler:
     head-update:
       traits:
         component_descriptor:
-          ocm_repository: gardener-public
+          ocm_repository_mappings:
+            - repository: europe-docker.pkg.dev/gardener-project/releases
         draft_release: ~
     pull-request:
       traits:
@@ -53,6 +54,12 @@ autoscaler:
       traits:
         version:
           preprocess: 'finalize'
+        component_descriptor:
+          ocm_repository: europe-docker.pkg.dev/gardener-project/releases
+        publish:
+          dockerimages:
+            cluster-autoscaler:
+              image: europe-docker.pkg.dev/gardener-project/releases/gardener/autoscaler/cluster-autoscaler
         release:
           nextversion: 'bump_minor'
         slack:
@@ -61,4 +68,3 @@ autoscaler:
             internal_scp_workspace:
               channel_name: 'C0170QTBJUW' # gardener-mcm
               slack_cfg_name: 'scp_workspace'
-        component_descriptor: ~