chore: update librarian to v0.28.0 (#20181)

This week's librarian version bump and `generate -all` command.

Author: suztomo

librarian.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 language: go
-version: v0.26.0
+version: v0.28.0
 repo: googleapis/google-cloud-go
 sources:
   discovery: