fix: honor -stderrthreshold when -logtostderr=true (#6847)#6869

ti-chi-bot[bot] merged 2 commits intopingcap:release-2.1pingcap/tidb-operator:release-2.1from

ti-chi-bot:cherry-pick-6847-to-release-2.1ti-chi-bot/tidb-operator:cherry-pick-6847-to-release-2.1Copy head branch name to clipboard