[CI] Add Github Action to notify devops of PRs labelled with A1-needsburnin (#6525)

* add burnin-label-notification.yml

* fix burnin-label-notification.yml

* fix burnin-label-notification.yml

* fix burnin-label-notification.yml

* fix burnin-label-notification.yml

* Update .github/workflows/burnin-label-notification.yml

Co-authored-by: Benjamin Kampmann <[email protected]>

Co-authored-by: Benjamin Kampmann <[email protected]>
Co-authored-by: Gav Wood <[email protected]>