chore(issue_template): Add reminder about the gitsync private repo

Author: NickLarsenNZ

.github/ISSUE_TEMPLATE/update-product-airflow.md

@@ -33,6 +33,7 @@ Add/Change/Remove anything that isn't applicable anymore
 - [ ] Check other operators (getting_started / kuttl / supported-versions) for usage of the versions. Add the PR(s) to the list below.
 - [ ] Ensure prerequisites are up to date (required-external-components.adoc).
 - [ ] Update the version in demos. Add the PR(s) to the list below.
+- [ ] Update the [stackable-airflow/dags](https://github.com/stackable-airflow/dags) private repository if needed.
 
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