Document security dashboard project limit

What does this MR do and why?

The security center has a limit of 100 projects.

This MR documents the limit.

There are two relevant limits.

In ee/app/models/users_security_dashboard_project.rb:

SECURITY_DASHBOARD_PROJECTS_LIMIT = 1000

In ee/app/assets/javascripts/security_dashboard/components/shared/filters/project_filter.vue

const SELECTED_PROJECTS_MAX_COUNT = 100;

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