Bump json-bigint and google-auth-library in /scripts/gcb2slack (#2869)

Bumps [json-bigint](https://github.com/sidorares/json-bigint) to 1.0.0 and updates ancestor dependency [google-auth-library](https://github.com/googleapis/google-auth-library-nodejs). These dependencies need to be updated together.


Updates `json-bigint` from 0.3.0 to 1.0.0
- [Release notes](https://github.com/sidorares/json-bigint/releases)
- [Commits](sidorares/json-bigint@v0.3.0...v1.0.0)

Updates `google-auth-library` from 4.2.6 to 8.7.0
- [Release notes](https://github.com/googleapis/google-auth-library-nodejs/releases)
- [Changelog](https://github.com/googleapis/google-auth-library-nodejs/blob/main/CHANGELOG.md)
- [Commits](googleapis/google-auth-library-nodejs@v4.2.6...v8.7.0)

---
updated-dependencies:
- dependency-name: json-bigint
  dependency-type: indirect
- dependency-name: google-auth-library
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

scripts/gcb2slack/package.json

@@ -6,6 +6,6 @@
   "dependencies": {
     "@google-cloud/functions-framework": "^1.3.2",
     "@slack/webhook": "^5.0.4",
-    "google-auth-library": "^4.2.6"
+    "google-auth-library": "^8.7.0"
   }
 }