Automation: Update backport github action trigger (#28352)
Automation: Update backport github action trigger (#28352)

It seems like GitHub has solved the problem of running github actions on PRs from forks with access to secrets. 

https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows/#improvements-for-public-repository-forks

If I change the event that triggers it to pull_request_target the action is run in the context of the base instead of the merged PR branch
grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana