ci: upgrade peter-evans/create-or-update-comment to v5

Updates 4 occurrences across 2 workflow files from v4 to v5.
v5 runs on Node 24; v4 runs on Node 20 which is
being deprecated on GitHub Actions runners.

See https://github.com/peter-evans/create-or-update-comment/releases/tag/v5.0.0

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: andrewhsu

.github/workflows/backport-command.yml

@@ -45,7 +45,7 @@ jobs:
         run: $SCRIPT_DIR/get_backport_type.sh
         shell: bash
       - name: Failed reaction
-        uses: peter-evans/create-or-update-comment@v4
+        uses: peter-evans/create-or-update-comment@v5
         if: failure()
         with:
           token: ${{ env.ACTIONS_BOT_TOKEN }}
@@ -265,15 +265,15 @@ jobs:
         shell: bash
       - name: Add reaction
         if: steps.pr_details.outcome == 'success' || steps.load_ai_handoff.outcome == 'success'
-        uses: peter-evans/create-or-update-comment@v4
+        uses: peter-evans/create-or-update-comment@v5
         with:
           token: ${{ env.ACTIONS_BOT_TOKEN }}
           repository: ${{ github.event.client_payload.github.payload.repository.full_name }}
           comment-id: ${{ github.event.client_payload.github.payload.comment.id }}
           reactions: hooray
       - name: Failed reaction
         if: always() && steps.pr_details.outcome == 'failure' && steps.load_ai_handoff.outcome != 'success'
-        uses: peter-evans/create-or-update-comment@v4
+        uses: peter-evans/create-or-update-comment@v5
         with:
           token: ${{ env.ACTIONS_BOT_TOKEN }}
           repository: ${{ github.event.client_payload.github.payload.repository.full_name }}

.github/workflows/buildkite-slash-commands.yml

@@ -49,7 +49,7 @@ jobs:
           buildkite_pipeline: redpanda
           command: ${{ github.event.client_payload.slash_command.command }}
       - name: Success reaction
-        uses: peter-evans/create-or-update-comment@v4
+        uses: peter-evans/create-or-update-comment@v5
         with:
           token: ${{ env.ACTIONS_BOT_TOKEN }}
           repository: ${{ github.event.client_payload.github.payload.repository.full_name }}