Bump container registry to v3.15.0

What does this MR do?

3.15.0 (2021-11-16)

Features

• gc: improve logging for artifact deletions (4d77d47)
• gc: record event type for manifest tasks queued due to an upload (1b2a534)
• handlers: temporarily log details of "find repository by path" queries (8a3fcca)
• storage: add Prometheus histogram metric for new blob uploads (8a9c4a0)

Related issues

Related to gitlab-org/container-registry#488 (closed) and gitlab-org/container-registry#487 (closed).

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated