Execute `security:validator` in an independent way

What does this MR do?

Starting with the current security release, we're now validating security merge requests in batches (https://gitlab.com/gitlab-org/release-tools/-/issues/369), meaning we validate security merge requests associated with a security issue. These changes are under a feature flag, and they're preventing our 'security:validate' cron task to be executed correctly.

To fix the above, we now introduce a new class (ProjectsValidator), that will be in charged to validate security merge requests associated with a security project. This is the task that will be called on security:validate

Related to https://gitlab.com/gitlab-org/release-tools/-/issues/426