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

[anna-geller]

Breaking Changes

Git Plugin: Default Branch Name Changed

The default branch for Git operations has been updated from kestra to main (PR #98). Update any workflows or integrations that implicitly reference the former default branch.

Secrets: Exception Thrown on Missing Secret

Fetching a non-existing secret using the secret() function now throws an exception instead of returning null, aligning the open-source behavior with the behavior in the Enterprise Edition (PR #6495).

Change State: Restart Downstream Task Runs

Manually changing a task run's status from Failed to a non-failed state (e.g. Success) via the Change state interface now restarts all downstream task runs, including subflows (PR #657).

Restarting Parent Flow with Failed Subflow

When restarting an execution, Subflow or ForEachItem tasks now restart the existing failed subflow execution rather than creating a new one. This behavior is configurable via the new restartBehavior enum property; setting it to NEW_EXECUTION retains the previous behavior (PR #6799; Issue #6722). A system.restarted: true label is added during restart for tracking, and the underlying subflow execution storage table is retained to avoid migration issues (scheduled for removal in v0.22).

Script Tasks: STDERR Logged at ERROR Level

Script tasks now log output sent to stderr at the ERROR level instead of WARNING (PR #6383; Issue #190).

Flows Created Before v0.9: Redeployment Required

Flows created before v0.9 and not updated since require editing or redeployment due to changes in source auto-generation (PR #6264).

[MilosPaunovic]

#6799 (comment)

[loicmathieu]

#6383

[loicmathieu]

#6264

But not sure it's worth mentionning