docs(artifact-registry): amend ADR-007 for the docker/oci format split

Context

ADR-007 still documents repositories.format as a three-value enum with slot 0 named container. The schema and code have carried four values (0=docker, 1=maven, 2=npm, 3=oci) since the ADR-009 docker/oci split landed in handbook!20526; the migration that widened the CHECK constraint documents the slot-0 reinterpretation.

This amendment updates the ERD annotation and the repositories bullet, and adds an amendment paragraph in ADR-007's established in-file style crediting the ADR-009 decision. Usage-data collection (ADR-012) is explicitly called out as unaffected: analytics reports both formats under one container dimension, a reporting mapping rather than a schema value.

The change syncs into the artifact-registry repo's docs/adr/ via its daily CI job after merge.