ci: fail the notify job when the announcement is rejected - #2040

Summary

This pull request sets errors: true on the release announcement step, so a webhook that rejects the post fails the notify job instead of logging an error and staying green.

• Suggested by @mwbrooks in slack-health-score#237. A missing secret already failed the step, but with the action's default errors: false a rejected post (say, a revoked webhook) only logged an error.
• The notify job is separate from the publish, so a red notify job doesn't affect the release. Re-run it with gh run rerun <id> --failed.
• The split-notify PRs (bolt-js#3148, node-slack-sdk#2853, slack-github-action#708, slack-skills-plugin#220) set it too, so every repo works the same way.

         uses: slackapi/slack-github-action@dcb1066f776dd043e64d0e8ba94ca15cc7e1875d # v4.0.0
         with:
+          errors: true
           webhook: ${{ secrets.SLACK_RELEASE_ANNOUNCEMENTS_WEBHOOK_URL }}

Testing

• On the next release, confirm the announcement posts and the notify job stays green.

Category

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs (Documents)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

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