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

In addition, switch testmachinery-Image to new location in Artifact-Registry.

[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.