In this PR:
- Sync googleapis/synthtool#1921
- Sync googleapis/synthtool#1922
- Sync googleapis/synthtool#1941

In this PR:
- Generate a selected list of libraries based on `library_name`

This PR is a preparation for improve nightly generation.

Note that we need to change the [new library generation
workflow](https://github.com/googleapis/google-cloud-java/blob/main/.github/workflows/generate_new_client_hermetic_build.yaml)
in google-cloud-java to accommodate this change because
`library_generation/generate_repo.py` has replaced
`--target-library-api-shortname` with `--target-library-names`.

In todays's nightly generation job
([log](https://github.com/googleapis/google-cloud-java/actions/runs/8457767424/job/23170489394)),
no actual generation is done.

I think this is because `main` method has been removed by #2598.

Port template change in
googleapis/synthtool#1945