ci-package-builder: Hardcode the releases to freeze MRs for

The pipeline really tries to ensure that updates to v2019 and v2020 go through the -updates/-security branches given that they are stable releases.

Unfortunately, for the pipeline the apertis/v2021 branch looks like it is stable already even if we haven't released it yet, as it only looks at the name.

I guess we really have to hardcode the list of stable branches since there's not way for it to figure automatically the to-be-stable-but-not-released-yet case.