Add age filter for scan result policies

What does this MR do and why?

This MR adds a new age filter for scan result policies. It's allowed to be set only together with previously existing status.

Screenshots or screen recordings

age_filter

Age filter dependent on Previously existing Status filter

age_filter_reactive

Severity filter removal

How to set up and validate locally

1. Ensure the GitLab Ultimate license is enabled
2. Navigate to a project/group => Security & Compliance => Policies => New policy => Scan result policy
3. Choose Security Scan
4. Verify the age filter is disabled
5. Add status filter, select Previously existing and any/all of the statuses.
6. Verify the age filter can be added now.
7. Change the status filter to New and verify the age filter gets deleted
8. Add the filters again, delete status filter and verify the age filter gets deleted with it

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 #397086 (closed)