workflows/backport: fix concurrent jobs cancelling each other

When a PR is merged and labeled afterwards - with a non-backport label -
the following will happen:
- The first backport job is triggered on the merge.
- The second backport job is triggered on the label event.
- The second job will cancel the first one due to the concurrency group.
- The second job will cancel itself because the label event didn't
contain a backport label.

Both jobs end up cancelled and no backport happens.

We made the backport action idempotent upstream a while ago, so we don't
need to cancel those actions. Instead, we'll run all of them -
subsequent actions running through will just stay silent anyway.

.github/workflows/backport.yml

@@ -9,10 +9,6 @@ on:
   pull_request_target:
     types: [closed, labeled]
 
-concurrency:
-  group: backport-${{ github.workflow }}-${{ github.event_name }}-${{ github.event.pull_request.number || github.run_id }}
-  cancel-in-progress: true
-
 permissions:
   contents: read
   issues: write