Add breaking change note for YAML validation change

The fleet plugin recently changed from using `js-yaml` to the `yaml` package in https://github.com/elastic/kibana/pull/252345 and with that, we introduced an accidental breaking change.

On Kibana versions 9.4+, the YAML validation is stricter (YAML 1.2 vs the old YAML 1.1), meaning that YAML that would previously pass validation is now rejected. This causes silent failures on upgrade to 9.4+ that the user is unaware of until they try to edit a policy. We are tracking an issue [here](https://github.com/elastic/kibana/issues/280822) to auto-detect this and improve it. In the meantime, we should update the docs to warn users that they will encounter silent breaking-changes and will not see the errors until they go in to edit policies.