Announcing Vitess 21

Vitess 21 introduces experimental support for atomic distributed transactions and recursive CTEs, enhancing query compatibility. VReplication gains reference table materialization and dynamic workflow configuration. Cluster management is improved with new VTOrc metrics for errant GTIDs. The throttler now supports multi-metric configurations for finer control over throttling behavior. Online DDL receives various improvements, including better instant DDL analysis and programmatic charset conversion. Backup and restore processes gain an experimental mysqlshell engine for logical backups and restores. VExplain is enhanced with `vexplain trace` for detailed query execution insights and `vexplain keys` for analyzing sharding key usage. The Vitess Kubernetes Operator adds horizontal autoscaling for VTGate pods and support for Kubernetes 1.31. VTAdmin introduces new pages for VReplication workflow creation/management and distributed transaction dashboards.