Bump axios and @slack/webhook in /scripts/gcb2slack

Bumps [axios](https://github.com/axios/axios) to 0.21.4 and updates ancestor dependency [@slack/webhook](https://github.com/slackapi/node-slack-sdk). These dependencies need to be updated together.


Updates `axios` from 0.18.1 to 0.21.4
- [Release notes](https://github.com/axios/axios/releases)
- [Changelog](https://github.com/axios/axios/blob/v0.21.4/CHANGELOG.md)
- [Commits](axios/axios@v0.18.1...v0.21.4)

Updates `@slack/webhook` from 5.0.2 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@5.0.2...@slack/webhook@5.0.4)

---
updated-dependencies:
- dependency-name: axios
  dependency-type: indirect
- dependency-name: "@slack/webhook"
  dependency-type: direct:production
...

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

Author: dependabot[bot]

scripts/gcb2slack/package-lock.json

@@ -5,7 +5,7 @@
   "main": "index.js",
   "dependencies": {
     "@google-cloud/functions-framework": "^1.3.2",
-    "@slack/webhook": "^5.0.2",
+    "@slack/webhook": "^5.0.4",
     "google-auth-library": "^4.2.6"
   }
 }