Improve performance for Projects::VulnerabilityFeedbackController#index

Why are we doing this work

Projects::VulnerabilityFeedbackController#index is disproportionately responsible for the error budget spend in groupthreat insights.

We want to reduce the number of request over the 1s threshold, which is around 5% as of this writing (17993/358327).

Relevant links

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

• backend implement standard limit/offset pagination on Project::VulnerabilityFeedback controller index, making sure to not paginate by default if pagination params are not provided
• backend count endpoint for Project::VulnerabilityFeedback controller index to enable the frontend to know how many pages worth of content to poll
• adjust the :index method urgency accordingly