Explicitly set the stable branches when pulling updates

Rather than relying on the format of the branch name, explicitly configure which branches should be considered stable. This allows us to land updates to apertis/v2022 directly before v2022.0 is released, rather than having all of them go to apertis/v2022-updates.

This is currently useful for downstream pulling updates from apertis/v2021 before their v2021.0 is released.

Task: T6873