Announcing Vitess 15

Vitess 15 is released, featuring the General Availability of VTOrc for automated monitoring and recovery, and VTAdmin for multi-cluster management. It also includes progress on VEP-4 for code reorganization and flag standardization, and VDiff v2 which is feature complete for resumable data migration checks. Performance enhancements include improved query plans for subqueries and derived tables, and updates to the benchmarking infrastructure.