Delete manifests in dependency proxy purge api

What does this MR do?

We have an API endpoint for clearing the Dependency Proxy cache. The Dependency Proxy stores container images from DockerHub. An image is made up of two types of files: blobs, and a manifest. Until recently, we only stored the blob files, but in !48845 (merged), we began storing manifests too.

When we purge the cache, what we are doing is simply deleting the files associated with a given group. The purge job deletes the blob files, but we need to update it to also delete the manifests. This MR adds that functionality.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #241639 (closed)