v3.21.6

What's Changed

• Fix CI build-ui job by @jmigueprieto in #256
• Fix flaky tests in HierarchicalForkJoinSubworkflowRerunSpec & HierarchicalForkJoinSubworkflowRestartSpec by @jmigueprieto in #257
• Handle duplicate event inserts gracefully when using Postgresql by @Robban1980 in #258
• Added (incubating) Java Client v3 project to conductor-clients dir by @jmigueprieto in #255
• Ignore conductor-clients changes in GitHub Action workflows by @jmigueprieto in #259
• Initial GitHub Action workflows to build Java client v4 by @jmigueprieto in #260
• Update Java Client v4 documentation by @jmigueprieto in #268
• Update conductor-architecture.png by @RizaFarheen in #269
• Documentation: Notes on Client v3 phase-out by @jmigueprieto in #271
• [Java Client v4] Gradle config refactoring by @jmigueprieto in #270
• [Java Client v4] External storage support + Event refactoring by @jmigueprieto in #274

Full Changelog: v3.21.5...v3.21.6