Improve message returned when adding non-Gold project to the Ops Dashboard

We currently return the error message of Some projects could not be added to the dashboard, when the project's license is not Gold. (This is not a problem on self-managed, as the entire feature is not available.)

We need to alter this message for this release, to indicate why the project could not be added. Otherwise users will think the feature is broken. This was discussed here: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7341#note_106577939

Proposal for message:

Unable to add <project name>. The Ops Dashboard is available for projects with a [Gold subscription](https://about.gitlab.com/pricing/).