Added lowercase to the actual con-reg api call

What does this MR do and why?

The deduplicated_size API only allows lowercase path in the uri, if the path is in uppercase the API results in 404.

This was identified as part of debugging a migration that backfilled container-registry-size for project_statistics here

An initial fix MR was deployed for the same.

But while checking a new backfill migration status on staging as part of https://gitlab.com/gitlab-org/gitlab/-/issues/368112, I saw uppercase characters still present in self_with_descendants API paths. This should fix that

This is important to merged before we "start" this migration on production.

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