Add users_security_dashboard_projects table

What does this MR do?

This MR is the first in a series that will implement the ability to add and manage projects in a user's security dashboard.

• Adds users_security_dashboard_projects table with references to users and projects
• Adds has_many: security_dashboard_projects to User

Issue: #33896 (closed) "Epic" issue: #6953 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.