workflows/backport: Copy security label in backport PRs

Since v1.2.0 the backport action supports copying labels from the source
to the backport PR. This is useful for copying the security label from
the original PR to the backport PR, so relevant security updates don't
get lost.

.github/workflows/backport.yml

@@ -27,6 +27,7 @@ jobs:
         uses: korthout/[email protected]
         with:
           # Config README: https://github.com/korthout/backport-action#backport-action
+          copy_labels_pattern: 'severity:\ssecurity'
           pull_description: |-
             Bot-based backport to `${target_branch}`, triggered by a label in #${pull_number}.