build(deps): bump @slack/webhook from 5.0.3 to 5.0.4 (#12944)

Bumps [@slack/webhook](https://github.com/slackapi/node-slack-sdk) from 5.0.3 to 5.0.4.
- [Release notes](https://github.com/slackapi/node-slack-sdk/releases)
- [Commits](https://github.com/slackapi/node-slack-sdk/compare/@slack/webhook@5.0.3...@slack/webhook@5.0.4)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

package.json

@@ -9,7 +9,7 @@
     "@babel/register": "^7.12.10",
     "@google-cloud/trace-agent": "^5.1.1",
     "@parse/node-apn": "^4.0.0",
-    "@slack/webhook": "^5.0.3",
+    "@slack/webhook": "^5.0.4",
     "accepts": "^1.3.5",
     "amazon-payments": "^0.2.8",
     "amplitude": "^5.1.4",