registry: update to v3

[idan-rahamim-lendbuzz]

Upgrade default registry image to v3

Fixes #406

Summary

Upgrades the default registry image from docker.io/library/registry:2 to docker.io/library/registry:3, aligning with the official kind documentation which has also moved to v3.

Changes

• Updated DefaultRegistryImageRef constant from registry:2 to registry:3
• Updated the Image field doc comment in Registry API type
• Updated CLI help text for --image flag
• Updated test fixtures to reflect registry v3 container metadata (entrypoint, command, config path)

Notes

• Registry v3 still exposes port 5000 and supports the same REGISTRY_STORAGE_DELETE_ENABLED env var, so no functional changes to ctlptl logic are needed
• The config path inside the container changed from /etc/docker/registry/config.yml to /etc/distribution/config.yml, but ctlptl doesn't set entrypoint/cmd — it inherits them from the image, so this is transparent
• Existing registries created with registry:2 will still be discovered via the dev.tilt.ctlptl.role=registry label filter