Update serverless requirement from ^3.19.0 to ^3.21.0 (#60)

Updates the requirements on [serverless](https://github.com/serverless/serverless) to permit the latest version.
- [Release notes](https://github.com/serverless/serverless/releases)
- [Changelog](https://github.com/serverless/serverless/blob/main/CHANGELOG.md)
- [Commits](serverless/serverless@v3.19.0...v3.21.0)

---
updated-dependencies:
- dependency-name: serverless
  dependency-type: direct:production
...

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

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

package.json

@@ -16,6 +16,6 @@
   },
   "homepage": "https://github.com/dragonraid/sls-action#readme",
   "dependencies": {
-    "serverless": "^3.19.0"
+    "serverless": "^3.21.0"
   }
 }