Bump @slack/webhook from 5.0.4 to 7.0.0 in /scripts/gcb2slack

Bumps [@slack/webhook](https://github.com/slackapi/node-slack-sdk) from 5.0.4 to 7.0.0.
- [Release notes](https://github.com/slackapi/node-slack-sdk/releases)
- [Commits](https://github.com/slackapi/node-slack-sdk/compare/@slack/webhook@5.0.4...@slack/webhook@7.0.0)

---
updated-dependencies:
- dependency-name: "@slack/webhook"
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

scripts/gcb2slack/package.json

@@ -5,7 +5,7 @@
   "main": "index.js",
   "dependencies": {
     "@google-cloud/functions-framework": "^1.3.2",
-    "@slack/webhook": "^5.0.4",
+    "@slack/webhook": "^7.0.0",
     "google-auth-library": "^9.0.0"
   }
 }