Add a note about the "List MR pipelines" endpoint only being available from 10.5.0

What does this MR do?

Makes it obvious that the "List MR pipelines" is only available from version 10.5.0.

This is a convention with all API endpoints as far as I can tell. Having a note like this would certainly save me few minutes of debugging.

The whole endpoint URL looks like this:

GET /projects/:id/merge_requests/:merge_request_iid/pipelines