Fix empty security dashboard for public projects

What does this MR do?

Project security dashboards were displaying no vulnerabilities for public projects, even when the latest pipeline reported vulnerabilities.

The issue was our Namespace#store_security_reports_available? method, which had no knowledge of the project or whether it was public, and only checked for availability based on the instance's license.

I've added Project#can_store_security_reports?, which calls into Namespace to check for license availability and also checks whether the project is public. Now public projects can get the security scanning they've been wanting!

#13422 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #13422 (closed)