Migrate to thin container for Owl Bot postprocessor

Now a thin container gcr.io/cloud-devrel-public-resources/owlbot-java-monorepo is available. Let's use it.

Migrate to thin container for Owl Bot postprocessor.

I use this issue as note to track the try-and-error.

# How Cloud Build runs the container?

```
# Run container listed in OwlBot.yaml:
- name: ${_CONTAINER}
dir: '${_PR_REPOSITORY}'
env:
- 'DEFAULT_BRANCH=$_DEFAULT_BRANCH'
```

https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/cloud-build/update-pr.yaml#L94