ci: use workflow-scoped GH PAT for backports (#21570)

This is necessary to allow backporting changes to GHA workflows, and
mirrors the token use in the CE->Ent merge workflow.

.github/workflows/backport-assistant.yml

@@ -27,7 +27,7 @@ jobs:
         env:
           BACKPORT_LABEL_REGEXP: "backport/(?P<target>\\d+\\.\\d+)"
           BACKPORT_TARGET_TEMPLATE: "release/{{.target}}.x"
-          GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN_WORKFLOW }}
           ENABLE_VERSION_MANIFESTS: true
   backport-ent:
     if: github.event.pull_request.merged && contains(join(github.event.pull_request.labels.*.name), 'backport/ent')