feat(gitlab): use forked_from_project for Merge Requests target project

Appian specific patch to always use the upstream (forked_from_project) when
dealing with Merge Requests or Issues.

This ensures that Renovate follows the standard development workflow where
branches are created against the "dev" fork and then Merge Requests
are opened against the forked (or "prod") fork.

In addition, we want the Dependency Dashboard created against the "prod"
fork, so any calls to the Issues API goes to the "prod" fork.