Bump aks-preview version to 19.0.0b21 for rollback feature release#9557

yanzhudd merged 1 commit intoAzure:mainAzure/azure-cli-extensions:mainfrom

InduSridhar:aks-agentpool-rollback-releaseInduSridhar/azure-cli-extensions:aks-agentpool-rollback-releaseCopy head branch name to clipboard