Remove usage of confidence from `.ordered` scope

`confidence` has been deprecated since `15.0`. It still isn't fully
removed, though we do have an issues to track that (see MR for links)

This commit just removes the usage of `confidence` in the `ordered`
scope of `Vulnerability::Finding` model. I am doing this as part of
the [epic][0] to delete the `vulnerability_finding_pipelines` table

I need to decouple a couple of sbom services from
`vulnerability_finding_pipelines`. A prerequisite of that decoupling
is improving the query performance of `.ordered`.

Removing `confidence` from the sort helps, and as it is deprecated
anyways this seems like a straightforward, quick win

[0]:https://gitlab.com/groups/gitlab-org/-/epics/11241

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MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/159532
Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/472760
Changelog: changed
EE: true