Replace slack-member-id with slack-group-id in .tekton files for release 1.6

This change updates the Tekton pipeline configuration to use slack-group-id
instead of slack-member-id parameter for Slack notifications.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: clyang82 <chuyang@redhat.com>

Author: clyang82

.tekton/postgres-exporter-globalhub-1-6-push.yaml

@@ -50,7 +50,7 @@ spec:
     value: "slack-notify-webhook"
   - name: slack-webhook-url-secret-key
     value: "slack-webhook-url"
-  - name: slack-member-id
+  - name: slack-group-id
     # Will mention the user in the slack message when the pipeline run failed; this is not required.
     # Please make sure replace the value with component owner's slack member id.
     value: "S09JCE3DF9N"