Add security policy scheduled scans concurrency limit

What does this MR do and why?

This MR adds a new application setting to define the number of concurrent security policy scheduled scans concurrency limit. We will use this application setting in a follow-up MR to limit CreatePipelineWorker's concurrency.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. Go to /admin/application_settings/ci_cd#js-ci-cd-settings
2. Verify the new application setting has the expected default value.
3. Update the value
4. Verify the new value was updated in the database using the rails console

ApplicationSetting.last.security_policy_scheduled_scans_max_concurrency