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.

tests/test_packaging_pipelines.py

@@ -43,10 +43,13 @@ def _retrying_on_exception(exception_type):
 
 def _find_stable_release_branch(project, prefix):
     branches = (b.name for b in project.branches.list(all=True))
-    releases = (b for b in branches if b.startswith(prefix))
+    releases = [b for b in branches if b.startswith(prefix)]
     stable = (b for b in releases if b[-4:-1] != "dev" and b[-3:] != "pre")
-    latest = sorted(stable, reverse=True)[0]
-    return latest
+    # do not pick the latest stable as it may not be actually released yet
+    # for instance, in 2021Q1 we have the apertis/v2021 branch but it
+    # should not be considered frozen yet since it has not been released
+    second_latest = sorted(stable, reverse=True)[1]
+    return second_latest
 
 
 def _monitor_pipeline_for_completion(pipeline):