"Security Dashboard unavailable" empty state

What does this MR do?

Introduces a separate empty state ("Security Dashboard unavailable"). Needed for the case when the "Group Overview content" feature is available but the user lacks the appropriate permissions for a group.

This is a part of the work being done for providing an option to set the Security Dashboard as a default view for groups.

What are the relevant issue numbers?

#7048 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer