pkg-merge-upstream-to-downstreams: Explicitly define stable branches

Currently the code pulling updates from upstream assumes that downstream branches ending with dev[0-9] or pre are not stable so updates can be directly landed there, while other downstream branches (for instance, apertis/v2021) are stable and updates should go through the updates or security repositories.

However, there's a time right before the first stable release is published that the "stable" branch is not stable yet. For instance, before the release of v2021.0 the apertis/v2021 should not have been considered stable and updates should have been landed directly there.

This is even more important for downstreams so that they can pull updates to their not-released-yet v2021 branch.

To do so, add a --stable parameter that accepts the list of branches to be considered "stable".

Task: T6873