Improve cadence validation for scan execution policy

Improve the cadence validation for the scan execution policy creation according to the supported cadences in our documentation.

Feature flag: scan_execution_policy_cadence_validation

Verification steps

1. Create a new project
2. Before enabling the feature flag, create a scan execution policy with a unsupported cadence using the yaml mode. A policy like:

type: scan_execution_policy
name: policy
description: ''
enabled: false
rules:
  - type: schedule
    branches:
      - '*'
    cadence: '*/15 * * * *'
actions:
  - scan: secret_detection

3. Enable the feature flag
4. Wait for the scheduled time and check the sidekiq logs for the error message "Invalid cadence"
5. Go to Secure > Policies
6. Try to save a policy with an unsupported cadence
7. Verify the error message Cadence is invalid is displayed