fix: gracefully handle unsupported schema v1 manifests during imports

Context

Related to gitlab#350920 (closed), found in gitlab#350920 (comment 917014676).

Problem

Schema v1 manifests were long ago deprecated (#213 (closed)) and support for them was later removed (!468 (merged)). Since then, no v1 manifests can be pushed to the registry, and any existing v1 manifests are treated as if they didn't exist.

While importing existing repositories we've run into a few migration failures due to encountering v1 manifests which were not gracefully skipped.

Solution

This MR ensures that the related code path is updated to ignore schema v1 manifests and continue with a repository import instead of halting it.