chore: add backstory to librarian.yaml and re-generate

[zhumin8]

Followup to #13351, add backstory back to librarian.yaml and re-generate code with librarian.

Manual add config to librarian.yaml and run:

V=$(go run github.com/googleapis/librarian/cmd/librarian@latest config get version)
go run github.com/googleapis/librarian/cmd/librarian@${V} install
export PATH=$PATH:$HOME/.cache/librarian/bin/java_tools/bin
go run github.com/googleapis/librarian/cmd/librarian@${V} generate backstory

Then manually reverted version change in gapic-libraries-bom/pom.xml, this is known limitation during partial release. See googleapis/librarian#6411.

Fixes googleapis/librarian#6409

[gemini-code-assist]

Code Review

This pull request updates the transport configuration for the java-backstory library to support both gRPC and HTTP/JSON, reflecting this change in the metadata and README. It also updates the Go package option in the proto files to point to cloud.google.com/go/backstory/backstorypb, adds Clirr ignored differences for a Protobuf upgrade, and registers the backstory library in librarian.yaml. There are no review comments, and the changes look correct, so I have no additional feedback to provide.

[zhumin8]

note: librarian defaults to grpc+rest as transport for generation. For this library, since it is proto only, these are purely documentation diff.