[sdk-platform-java] `update_generation_config.sh` does not consider existing closed PRs when checking for branch existence

For example: https://github.com/googleapis/java-pubsub/blob/ce7cb09ac5e86953815e2ead9ae9202e9b034ba6/.github/scripts/update_generation_config.sh#L109 is used to check whether we have an **open** PR with the target branch name, but unluckily java-pubsub had a **closed** PR with that branch name, so the whole action failed due to a `git push` that conflicted with the remote's contents.

An easy workaround: search for closed PRs (in this case it was [this one](https://github.com/googleapis/java-pubsub/pull/2581)) and run the action again.