Add schema validation for approvals_required

What does this MR do and why?

This MR adds schema validation for approvals_required. There is already a model validation for approval rules, so by adding this validation into the schema, we can prevent situations where no merge request approval rules are created because the value is > 100 and thus not in line with the model validations.

Screenshots or screen recordings

How to set up and validate locally

1. Go to Secure -> Policies
2. Create a new scan result policy. You can switch to the .yaml mode and use the following:

type: scan_result_policy
name: Approvals
description: ''
enabled: true
rules:
  - type: scan_finding
    branches: []
    scanners:
      - secret_detection
    vulnerabilities_allowed: 0
    severity_levels:
      - critical
      - high
    vulnerability_states:
      - new_needs_triage
      - new_dismissed
actions:
  - type: require_approval
    approvals_required: 101
    user_approvers_ids:
      - 4

1. There should be a validation error for approvals_required, as can be seen on the screenshot

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 #409469 (closed)