Rename VulnerabilityFeedbackModule namespace for consitency

In ee/app/services, VulnerabilityFeedbackModule is the only namespace with Module in the name.

It should be updated to VulnerabilityFeedback, along with any references to it, for consistency.

See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11963#note_166190532 for more context.