Resolve "Update error message displayed when adding an unsupported project to Ops Dashboard"

What does this MR do?

Updates the error message displayed when adding an unsupported project to Ops Dashboard, to reflect the fact that public projects are supported.

Before	After

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8444

docs MR here: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8710

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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

Closes #8444 (closed)