With the new release identification / naming schema [1] (like:
2023.1 Antelope) new stable branch naming was introduced (like:
stable/2023.1). This was not handled in check_branch_sha method
in validate.py, as the method still searches stable/$series.
This patch fixes this by reading the 'release-id' field from
series_status.yaml if present and uses it as stable/<release-id>
for the branch search.
[1] https://governance.openstack.org/tc/reference/release-naming.html
Change-Id: I014b1d6dc561be4db0fc8faa85fb3133e851acfc
fb08afc1fa