Fail when issuable_meta_data is called on an unlimited collection

What does this MR do?

Fix a performance problem with the issues API, where we were loading metadata for the un-paginated collection.

Are there points in the code the reviewer needs to double check?

The condition in Gitlab::IssuableMetadata#issuable_meta_data could maybe be clearer?

Why was this MR needed?

We were trying to load metadata for all of a project's (or group's) issues, even when only getting a page's worth of issues 🤦

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/39845.