Remove deprecated markdow preview routes usage
What does this MR do and why?
As part of #442218 for project and group we want to move to the new
routes scoped under the /-/
pattern. Frontend already uses the new route, but this MR takes care of places where the backend provides the URL to the frontend. After confirming there's no usage of the route, we can remove it in the next release
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Related to #442218
Edited by Mario Celi