Release of Vitess v12.0.4

Changelog

Bug fixes

Cluster management

• Fix the race between PromoteReplica and replication manager tick #9859

Query Serving

• Route explain tab plan to the proper Keyspace #10028
• Diverse fixes to UNION planning and execution #10344

The release includes 5 commits (excluding merges)

Thanks to all our contributors: @GuptaManan100, @frouioui, @systay