[Post-Release] Add Breaking Changes for 0.21 to GitHub Release Notes

[anna-geller]

Core Task

• Subflow restart default behavior

Git plugin

The default branch name has changed from kestra to main.

Internal Storage Configuraiton

#3933

Secrets

#6495

Restarting parent flow with failed subflow

When restarting an execution, Subflowor ForEachItem tasks will restart the existing failed subflow instead of creating new ones.

This behavior is manageable via a new restartBehavior Enum property. Configure it to NEW_EXECUTION if you want to keep the previous behavior of creating new subflow executions on restart.

#6571 (comment)

[MilosPaunovic]

#6799 (comment)