Disable practracker in CI, when a release becomes a release candidate

• we get practracker on master, where it is most useful
• we keep practracker in alphas, where we still get some large code changes
• we don't have to worry about practracker for backports:
  • we rely on the existing backport triage process to maintain code quality in backport branches
  • we don't need practracker for backport branches, because we (usually) don't make large changes to backport branches