Bump protobuf in /PlayAssetDelivery/BundletoolScriptSample

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.12.4 to 3.18.3.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.12.4...v3.18.3)

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

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

Author: dependabot[bot]

PlayAssetDelivery/BundletoolScriptSample/requirements.txt

@@ -1 +1 @@
-protobuf==3.12.4
+protobuf==3.18.3