chore: update librarian to v0.28.0

[suztomo]

Librarian version upgrade and the generate -all command:

(.venv) suztomo@suztomo:~/librarian-2026/google-cloud-java$ go run github.com/googleapis/librarian/cmd/librarian@latest update version
(.venv) suztomo@suztomo:~/librarian-2026/google-cloud-java$ V=$(go run github.com/googleapis/librarian/cmd/librarian@latest config get version)
(.venv) suztomo@suztomo:~/librarian-2026/google-cloud-java$ echo $V
v0.28.0
(.venv) suztomo@suztomo:~/librarian-2026/google-cloud-java$ go run github.com/googleapis/librarian/cmd/librarian@${V} generate --all

This didn't generate any diff.

[gemini-code-assist]

Code Review

This pull request updates the version configuration in librarian.yaml from v0.27.1 to v0.28.0. There are no review comments, and I have no feedback to provide.