Validate vulnerability_age for presence of previously existing states

What does this MR do and why?

This MR adds validation for vulnerability_age and checks for presence of previously existing vulnerability_states. It's based on the requirement in the epic &6826 (closed):

Age of vulnerability can only be selected when creating a policy based on previously detected vulnerabilities.

Screenshots or screen recordings

How to set up and validate locally

1. Create a new scan result policy
2. Switch to the .yaml mode and use the following YAML:

type: scan_result_policy
name: Age
description: ''
enabled: true
rules:
  - type: scan_finding
    scanners: []
    vulnerabilities_allowed: 0
    severity_levels: []
    vulnerability_states:
      - new_needs_triage
    branch_type: protected
    vulnerability_age:
      operator: greater_than
      value: 2
      interval: week
actions:
  - type: require_approval
    approvals_required: 1
    user_approvers_ids:
      - 1 # Replace with an existing user ID

1. Click configure with merge request
2. Verify that error appears "Vulnerability age requires previously existing vulnerability states"
3. Add/change vulnerability_states to include detected
4. There should be no error

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)