actions: add backporting bot

With new event 'pull_request_target' it is now possible to run the
backporting action that we used to have in the past as a Github app.

Use labels for backporting as before.

See https://github.com/tibdex/backport for more details.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
1 file changed