google-cloud-java: update partial release py script to update librarian.yaml

In partial release guide [here](https://g3doc.corp.google.com/company/teams/cloud-sdk/client-libraries/team/java/playbooks/releasing/index.md?LIBRARY_SHORTNAME=asset&MAJOR_MINOR_VERSION=1.45#google-cloud-java-partial-releases), we added manual steps to update librarian.yaml version and released_version.
To make this less error prone and streamline the process, update the existing python script that updates version.txt, let it also update librarian.yaml versions.
cc. @blakeli0

Start with the existing Python script that updates version.txt and review the partial release guide for the expected librarian.yaml changes. Update the script so it changes both librarian.yaml version fields, version and released_version, and verify that the generated release configuration matches the guide.

Written by the indexing model from the issue text.