fix(infracost): replace deprecated set-output with GITHUB_OUTPUT

The Slack-message step used the legacy ::set-output workflow command,
which GitHub has deprecated and will disable. Write to $GITHUB_OUTPUT
instead, using a run-id-scoped heredoc delimiter for the multi-line
slack-message value.

Author: clouddrove-factory[bot]

.github/workflows/infracost.yml

@@ -87,8 +87,12 @@ jobs:
       - name: 📨 Generate Slack message
         id: infracost-slack
         run: |
-          echo "::set-output name=slack-message::$(infracost output --path=/tmp/infracost.json --format=slack-message --show-skipped)"
-          echo "::set-output name=diffTotalMonthlyCost::$(jq '(.diffTotalMonthlyCost // 0) | tonumber' /tmp/infracost.json)"
+          {
+            echo "slack-message<<EOF_${{ github.run_id }}"
+            infracost output --path=/tmp/infracost.json --format=slack-message --show-skipped
+            echo "EOF_${{ github.run_id }}"
+          } >> "$GITHUB_OUTPUT"
+          echo "diffTotalMonthlyCost=$(jq '(.diffTotalMonthlyCost // 0) | tonumber' /tmp/infracost.json)" >> "$GITHUB_OUTPUT"
 
       - name: 🚀 Send cost estimate to Slack
         uses: slackapi/slack-github-action@v3