Update documentation to reflect that the ops dashboard supports public projects

What does this MR do?

Updates the Operations Dashboard documentation to reflect that public projects are supported on Gitlab.com

Related Issues

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

Feature Change MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8672

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards