Add operational_vulnerabilities_filters feature flag

What does this MR do and why?

Hide cluster filter behind a feature flag

• there are some backend issues with the cluster filter on the operational vulnerabilities report (https://gitlab.slack.com/archives/CU9V380HW/p1655842412482469)
• hide it behind a feature flag instead of reverting it
• update tests

Changelog: added

EE: true

Screenshots or screen recordings

W/ ff off	w/ ff on

How to set up and validate locally

1. Upload a GitLab Ultimate license
2. Navigate to a project => Security & Compliance => Vulnerability Report => Operational Vulnerabilities
3. Verify Cluster filter is hidden
4. Enable the :operational_vulnerabilities_filters feature flag
5. Verify Cluster filter is shown

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #365986 (closed)