Ensure that we don't display triggers with "flow source not found"

[anna-geller]

Feature description

Automatically remove such triggers — it's quite confusing to display them if the flow no longer exists - we can consider some background cleanup process to avoid showing them

To reproduce: on the preview, we have tens of those zombie triggers https://preview-ee-kafka.kestra.io/ui/admin/triggers?size=100&page=1

[anna-geller]

From the team:

There should not be a trigger if the flow is deleted, we should delete the trigger so this may be a bug. We could add defensive code to asynchronously delete triggers that are no longer relevant when they are queried (and fix the root cause). the main issue is probably that somewhere in the code, we are deleting flow, without deleting triggers